feat: Add host-based assistant avatars (Neko/Mochi)

- Add avatar field to hosts.json config for Neko and Mochi
- Create HostContext to provide host config to components
- Display host avatar and name in chat messages instead of generic Claude
- Add user avatar and first name to user messages
- Include avatar in /api/hosts endpoint response
- Fix appendFileSync import for debug logging

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

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

backend/public/claude.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+  <defs>
+    <linearGradient id="grad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#f97316;stop-opacity:1" />
+      <stop offset="100%" style="stop-color:#ea580c;stop-opacity:1" />
+    </linearGradient>
+  </defs>
+  <rect width="100" height="100" rx="20" fill="url(#grad)"/>
+  <text x="50" y="68" font-family="Arial, sans-serif" font-size="50" font-weight="bold" fill="white" text-anchor="middle">C</text>
+</svg>