Add frontend CI workflow, error boundary, and VITE_API_URL support

- .github/workflows/frontend.yml: type-check + build on push/PR
- ErrorBoundary component wraps App with catch + retry UI
- API client reads VITE_API_URL env var for non-same-origin deploys
- vite-env.d.ts declares ImportMetaEnv type

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

.github/workflows/frontend.yml

@@ -0,0 +1,37 @@
+name: Frontend CI
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'frontend/**'
+      - '.github/workflows/frontend.yml'
+  pull_request:
+    branches: [main]
+    paths:
+      - 'frontend/**'
+      - '.github/workflows/frontend.yml'
+  workflow_dispatch:
+
+concurrency:
+  group: frontend-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: frontend
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: 'npm'
+          cache-dependency-path: frontend/package-lock.json
+
+      - run: npm ci
+      - run: npx tsc --noEmit
+      - run: npm run build