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/Dockerfile

@@ -25,8 +25,8 @@ COPY . .
 # Build the app
 RUN npm run build
 
-# Production stage
-FROM nginx:alpine
+# Production stage - using nginx with brotli support
+FROM fholzer/nginx-brotli:v1.28.0
 
 # Copy built files
 COPY --from=builder /app/dist /usr/share/nginx/html
@@ -36,4 +36,5 @@ COPY nginx.conf /etc/nginx/conf.d/default.conf
 
 EXPOSE 80
 
-CMD ["nginx", "-g", "daemon off;"]
+# fholzer/nginx-brotli has ENTRYPOINT ["nginx"], so only pass args
+CMD ["-g", "daemon off;"]