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>

backend/.npmrc

@@ -0,0 +1,5 @@
+fetch-timeout=300000
+fetch-retries=3
+fetch-retry-factor=2
+fetch-retry-mintimeout=10000
+fetch-retry-maxtimeout=60000