Add pre-commit hooks for linting and formatting

Set up pre-commit framework with ruff (backend) and ESLint/Prettier/tsc
(frontend) hooks to catch issues locally before CI. Auto-format all
frontend files with Prettier to comply with the new check.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -45,6 +45,9 @@ jobs:
       - name: Lint
         run: npm run lint
         working-directory: frontend
+      - name: Check formatting
+        run: npx prettier --check "src/**/*.{ts,tsx,css,json}"
+        working-directory: frontend
       - name: Type check
         run: npx tsc -b
         working-directory: frontend