Fix frontend build: compile React bundle properly

- Build TypeScript/React to JavaScript bundle
- Serve compiled main.js and main.css
- Update HTML to load compiled assets
- Add build step to Dockerfile

Co-Authored-By: Claude Sonnet 4.5 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -11,6 +11,9 @@ RUN bun install --frozen-lockfile --production
 # Copy source code
 COPY . .
 
+# Build the React bundle
+RUN bun run build
+
 # Expose port
 EXPOSE 3001