feat(ui): add custom theme override system

Create custom-theme.css for theme overrides that won't conflict
with upstream updates. The file loads after globals.css, so its
CSS variables take precedence.

This approach ensures:
- Zero merge conflicts on git pull (new file, not in upstream)
- Theme persists across upstream updates
- Easy to modify without touching upstream code

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

ui/src/main.tsx

@@ -3,6 +3,7 @@ import { createRoot } from 'react-dom/client'
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
 import App from './App'
 import './styles/globals.css'
+import './styles/custom-theme.css' // Custom theme overrides (safe from upstream conflicts)
 
 const queryClient = new QueryClient({
   defaultOptions: {