perf: Major performance overhaul with virtual scrolling and context splitting

Phase 1 - Virtual Scrolling:
- Add @tanstack/react-virtual for efficient message list rendering
- Only render visible messages instead of entire history
- Fix auto-scroll using native scrollTop instead of unreliable virtualizer

Phase 2 - Context Optimization:
- Split monolithic SessionContext into 4 specialized contexts
- MessagesContext, SessionsContext, SettingsContext, UIContext
- Prevents unnecessary re-renders across unrelated components

Phase 3 - Compression & Cleanup:
- Enable Brotli compression (~23% smaller than gzip)
- Switch to fholzer/nginx-brotli:v1.28.0 image
- Add automatic upload cleanup for idle sessions

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

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

frontend/src/App.jsx

@@ -1,7 +1,10 @@
 import { useState, useCallback, useEffect } from 'react';
 import { AuthProvider, useAuth } from './contexts/AuthContext';
-import { SessionProvider, useSessionManager } from './contexts/SessionContext';
 import { HostProvider } from './contexts/HostContext';
+import { SettingsProvider } from './contexts/SettingsContext';
+import { UIProvider, useUI } from './contexts/UIContext';
+import { MessagesProvider } from './contexts/MessagesContext';
+import { SessionsProvider, useSessions } from './contexts/SessionsContext';
 import { Sidebar } from './components/Sidebar';
 import { TabBar } from './components/TabBar';
 import { ChatPanel } from './components/ChatPanel';
@@ -10,13 +13,8 @@ import { LoginPage } from './components/LoginPage';
 import { Menu, Loader2 } from 'lucide-react';
 
 function AppContent() {
-  const {
-    sessions,
-    tabOrder,
-    splitSessions,
-    focusedSessionId,
-    createSession,
-  } = useSessionManager();
+  const { sessions, createSession } = useSessions();
+  const { tabOrder, splitSessions, focusedSessionId } = useUI();
 
   const [sidebarOpen, setSidebarOpen] = useState(true);
 
@@ -130,9 +128,15 @@ function AuthenticatedApp() {
 
   return (
     <HostProvider>
-      <SessionProvider>
-        <AppContent />
-      </SessionProvider>
+      <SettingsProvider>
+        <UIProvider>
+          <MessagesProvider>
+            <SessionsProvider>
+              <AppContent />
+            </SessionsProvider>
+          </MessagesProvider>
+        </UIProvider>
+      </SettingsProvider>
     </HostProvider>
   );
 }