feat: Add copy-to-clipboard buttons for code blocks and file paths

Add copy functionality to improve UX when working with code:
- Code blocks: pill-style "Copy" button appears on hover (top-right)
- File paths in Edit/Read/Write tools: icon button on hover
- Long inline code (>40 chars): small icon button on hover

Uses Clipboard API with visual feedback (green checkmark on success).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/CopyButton.jsx

@@ -0,0 +1,124 @@
+import { memo, useState, useCallback } from 'react';
+import { Copy, Check } from 'lucide-react';
+
+/**
+ * Reusable copy-to-clipboard button with feedback
+ *
+ * Variants:
+ * - "icon" (default): Small icon button, good for inline use
+ * - "pill": Pill-shaped button with text, good for code blocks
+ */
+export const CopyButton = memo(function CopyButton({
+  text,
+  variant = 'icon',
+  className = '',
+  size = 'sm' // 'xs', 'sm', 'md'
+}) {
+  const [copied, setCopied] = useState(false);
+
+  const handleCopy = useCallback(async (e) => {
+    e.stopPropagation();
+    e.preventDefault();
+
+    try {
+      await navigator.clipboard.writeText(text);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 2000);
+    } catch (err) {
+      console.error('Failed to copy:', err);
+    }
+  }, [text]);
+
+  const sizeClasses = {
+    xs: 'w-3.5 h-3.5',
+    sm: 'w-4 h-4',
+    md: 'w-5 h-5',
+  };
+
+  const iconSize = sizeClasses[size] || sizeClasses.sm;
+
+  if (variant === 'pill') {
+    return (
+      <button
+        onClick={handleCopy}
+        className={`
+          flex items-center gap-1.5 px-2 py-1 rounded-md text-xs font-medium
+          transition-all duration-200
+          ${copied
+            ? 'bg-green-500/20 text-green-400'
+            : 'bg-dark-700 hover:bg-dark-600 text-dark-400 hover:text-dark-200'
+          }
+          ${className}
+        `}
+        title={copied ? 'Copied!' : 'Copy to clipboard'}
+      >
+        {copied ? (
+          <>
+            <Check className="w-3.5 h-3.5" />
+            <span>Copied</span>
+          </>
+        ) : (
+          <>
+            <Copy className="w-3.5 h-3.5" />
+            <span>Copy</span>
+          </>
+        )}
+      </button>
+    );
+  }
+
+  // Default: icon variant
+  return (
+    <button
+      onClick={handleCopy}
+      className={`
+        p-1 rounded transition-all duration-200
+        ${copied
+          ? 'text-green-400 bg-green-500/20'
+          : 'text-dark-500 hover:text-dark-300 hover:bg-dark-700'
+        }
+        ${className}
+      `}
+      title={copied ? 'Copied!' : 'Copy to clipboard'}
+    >
+      {copied ? (
+        <Check className={iconSize} />
+      ) : (
+        <Copy className={iconSize} />
+      )}
+    </button>
+  );
+});
+
+/**
+ * Wrapper component that adds a copy button to any content
+ * Useful for wrapping code blocks, file paths, etc.
+ */
+export const CopyWrapper = memo(function CopyWrapper({
+  text,
+  children,
+  className = '',
+  buttonPosition = 'top-right', // 'top-right', 'top-left', 'inline-right'
+  buttonVariant = 'icon',
+  showOnHover = true,
+}) {
+  const positionClasses = {
+    'top-right': 'absolute top-2 right-2',
+    'top-left': 'absolute top-2 left-2',
+    'inline-right': 'ml-2 inline-flex',
+  };
+
+  const isAbsolute = buttonPosition !== 'inline-right';
+
+  return (
+    <div className={`${isAbsolute ? 'relative group' : 'inline-flex items-center'} ${className}`}>
+      {children}
+      <div className={`
+        ${positionClasses[buttonPosition]}
+        ${showOnHover && isAbsolute ? 'opacity-0 group-hover:opacity-100 transition-opacity duration-200' : ''}
+      `}>
+        <CopyButton text={text} variant={buttonVariant} />
+      </div>
+    </div>
+  );
+});

frontend/src/components/MessageList.jsx

@@ -5,7 +5,8 @@ import {
   User, Bot, Terminal, CheckCircle, AlertCircle, Info,
   FileText, Search, FolderSearch, Pencil, FilePlus, Globe,
   ChevronDown, ChevronRight, Play, ArrowDown,
-  ClipboardList, Brain, Paperclip, Image, HelpCircle, Zap, Command
+  ClipboardList, Brain, Paperclip, Image, HelpCircle, Zap, Command,
+  Copy, Check
 } from 'lucide-react';
 import ReactMarkdown from 'react-markdown';
 import remarkGfm from 'remark-gfm';
@@ -22,6 +23,49 @@ const SyntaxHighlighter = lazy(() =>
 // Import style separately (small JSON, OK to load eagerly for consistency)
 import { oneDark } from 'react-syntax-highlighter/dist/esm/styles/prism';
 
+// Copy button component for code blocks
+const CopyButton = memo(function CopyButton({ text, variant = 'icon', className = '' }) {
+  const [copied, setCopied] = useState(false);
+
+  const handleCopy = useCallback(async (e) => {
+    e.stopPropagation();
+    e.preventDefault();
+    try {
+      await navigator.clipboard.writeText(text);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 2000);
+    } catch (err) {
+      console.error('Failed to copy:', err);
+    }
+  }, [text]);
+
+  if (variant === 'pill') {
+    return (
+      <button
+        onClick={handleCopy}
+        className={`flex items-center gap-1.5 px-2 py-1 rounded-md text-xs font-medium transition-all duration-200
+          ${copied ? 'bg-green-500/20 text-green-400' : 'bg-dark-700 hover:bg-dark-600 text-dark-400 hover:text-dark-200'}
+          ${className}`}
+        title={copied ? 'Copied!' : 'Copy to clipboard'}
+      >
+        {copied ? <><Check className="w-3.5 h-3.5" /><span>Copied</span></> : <><Copy className="w-3.5 h-3.5" /><span>Copy</span></>}
+      </button>
+    );
+  }
+
+  return (
+    <button
+      onClick={handleCopy}
+      className={`p-1 rounded transition-all duration-200
+        ${copied ? 'text-green-400 bg-green-500/20' : 'text-dark-500 hover:text-dark-300 hover:bg-dark-700'}
+        ${className}`}
+      title={copied ? 'Copied!' : 'Copy to clipboard'}
+    >
+      {copied ? <Check className="w-4 h-4" /> : <Copy className="w-4 h-4" />}
+    </button>
+  );
+});
+
 // Fallback component for code blocks while SyntaxHighlighter loads
 const CodeFallback = memo(function CodeFallback({ children }) {
   return (
@@ -31,9 +75,17 @@ const CodeFallback = memo(function CodeFallback({ children }) {
   );
 });
 
-// Wrapper for lazy-loaded SyntaxHighlighter with Suspense
-const LazyCodeBlock = memo(function LazyCodeBlock({ language, style, customStyle, children, ...props }) {
+// Wrapper for lazy-loaded SyntaxHighlighter with Suspense and Copy button
+const LazyCodeBlock = memo(function LazyCodeBlock({ language, style, customStyle, children, showCopy = true, ...props }) {
+  const codeString = String(children).replace(/\n$/, '');
+
   return (
+    <div className="relative group">
+      {showCopy && (
+        <div className="absolute top-2 right-2 opacity-0 group-hover:opacity-100 transition-opacity duration-200 z-10">
+          <CopyButton text={codeString} variant="pill" />
+        </div>
+      )}
       <Suspense fallback={<CodeFallback>{children}</CodeFallback>}>
         <SyntaxHighlighter
           language={language}
@@ -41,9 +93,10 @@ const LazyCodeBlock = memo(function LazyCodeBlock({ language, style, customStyle
           customStyle={customStyle}
           {...props}
         >
-        {children}
+          {codeString}
         </SyntaxHighlighter>
       </Suspense>
+    </div>
   );
 });
 
@@ -563,7 +616,11 @@ const Message = memo(function Message({ message, onSendMessage, hostConfig }) {
                   components={{
                     code({ node, inline, className, children, ...props }) {
                       const match = /language-(\w+)/.exec(className || '');
-                      return !inline && match ? (
+                      const codeString = String(children).replace(/\n$/, '');
+
+                      // Block code with language
+                      if (!inline && match) {
+                        return (
                           <LazyCodeBlock
                             style={oneDark}
                             language={match[1]}
@@ -576,9 +633,27 @@ const Message = memo(function Message({ message, onSendMessage, hostConfig }) {
                             }}
                             {...props}
                           >
-                          {String(children).replace(/\n$/, '')}
+                            {codeString}
                           </LazyCodeBlock>
-                      ) : (
+                        );
+                      }
+
+                      // Long inline code (>40 chars) - show copy button on hover
+                      if (inline && codeString.length > 40) {
+                        return (
+                          <span className="relative inline-flex items-center group/code">
+                            <code className={`${className || ''} bg-dark-800 px-1.5 py-0.5 rounded text-orange-300`} {...props}>
+                              {children}
+                            </code>
+                            <span className="opacity-0 group-hover/code:opacity-100 transition-opacity duration-200 ml-1">
+                              <CopyButton text={codeString} size="xs" />
+                            </span>
+                          </span>
+                        );
+                      }
+
+                      // Regular inline code
+                      return (
                         <code className={className} {...props}>
                           {children}
                         </code>
@@ -957,9 +1032,12 @@ const ToolUseCard = memo(function ToolUseCard({ tool, input, result, onSendMessa
       return (
         <div className="space-y-2">
           {/* File header */}
-          <div className="flex items-center gap-2 text-xs">
+          <div className="flex items-center gap-2 text-xs group/path">
             <span className="text-dark-500">File:</span>
             <code className="text-orange-400">{input.file_path}</code>
+            <span className="opacity-0 group-hover/path:opacity-100 transition-opacity">
+              <CopyButton text={input.file_path} size="xs" />
+            </span>
           </div>
 
           {/* Diff view */}
@@ -1066,9 +1144,12 @@ const ToolUseCard = memo(function ToolUseCard({ tool, input, result, onSendMessa
     if (tool === 'Read') {
       return (
         <div className="space-y-2">
-          <div className="flex items-center gap-2">
+          <div className="flex items-center gap-2 group/path">
             <span className="text-xs text-dark-500">File:</span>
             <code className="text-xs text-blue-400">{input.file_path}</code>
+            <span className="opacity-0 group-hover/path:opacity-100 transition-opacity">
+              <CopyButton text={input.file_path} size="xs" />
+            </span>
           </div>
           {(input.offset || input.limit) && (
             <div className="flex items-center gap-4">
@@ -1197,9 +1278,12 @@ const ToolUseCard = memo(function ToolUseCard({ tool, input, result, onSendMessa
 
       return (
         <div className="space-y-2">
-          <div className="flex items-center gap-2 text-xs">
+          <div className="flex items-center gap-2 text-xs group/path">
             <span className="text-dark-500">File:</span>
             <code className="text-green-400">{input.file_path}</code>
+            <span className="opacity-0 group-hover/path:opacity-100 transition-opacity">
+              <CopyButton text={input.file_path} size="xs" />
+            </span>
             <span className="text-dark-600">•</span>
             <span className="text-dark-500">{lines} lines</span>
           </div>