perf: Add performance optimizations and Tanuki avatar

Frontend:
- Add memoization to MessageList with custom comparison function
- Implement incremental caching for processedMessages to avoid O(n) rebuilds during streaming
- Wrap Message component with memo()
- Add better error handling for file uploads in SessionContext

Backend:
- Improve upload error handling with proper response checks

Infrastructure:
- Add client_max_body_size 100m to nginx for file uploads
- Add Tanuki avatar (optimized 256x256, 77KB)

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

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

frontend/nginx.conf

@@ -19,6 +19,8 @@ server {
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         # Always HTTPS since NPM handles SSL termination (required for secure cookies)
         proxy_set_header X-Forwarded-Proto https;
+        # Allow file uploads up to 100MB
+        client_max_body_size 100m;
     }
 
     # Auth routes proxy