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/nginx.conf

@@ -4,10 +4,15 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
-    # Gzip
+    # Gzip compression
     gzip on;
     gzip_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript;
 
+    # Brotli compression (requires fholzer/nginx-brotli image)
+    brotli on;
+    brotli_comp_level 6;
+    brotli_types text/plain text/css application/json application/javascript text/xml application/xml application/xml+rss text/javascript application/wasm;
+
     # Backend API proxy (same network namespace via netbird-client)
     # Using 127.0.0.1 instead of localhost to force IPv4 (avoids IPv6 connection issues)
     # Note: proxy_pass without URI preserves URL encoding (important for paths with %2F)
@@ -47,6 +52,9 @@ server {
         proxy_set_header X-Forwarded-Proto https;
         proxy_read_timeout 86400;
         proxy_send_timeout 86400;
+        # Disable buffering for real-time streaming
+        proxy_buffering off;
+        proxy_cache off;
     }
 
     # SPA routing for frontend