feat: React admin UI migration — React 19 + TypeScript + Traffic/Uptime tabs

Replace 1,928-line monolithic vanilla JS admin SPA with React 19 + TypeScript 5
application. Adds Traffic and Uptime tabs backed by new Rust API endpoints.

- Vite + vite-plugin-singlefile: single dist/index.html with inlined JS/CSS
- Custom closeBundle plugin preserves __CSP_NONCE__ placeholder for Rust injection
- React Query v5 for server state, Zustand v5 for auth + WS connection state
- All existing backend contracts preserved (CSRF, WS auth, sessionStorage key)
- New: GET /admin/api/traffic (route load, P95, req/min, time series)
- New: GET /admin/api/uptime (proxy start time, per-backend 30d history)
- New: health_checks SQLite table + background Tokio health-checker (30s probe)
- BackendHealthChanged WS event for immediate Uptime tab refresh on status flip
- Dockerfile gains Node.js Stage 1 for frontend build
- CI gains frontend job (tsc + lint + build) before Rust test job

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

.github/workflows/ci.yml

@@ -11,10 +11,43 @@ env:
   CARGO_TERM_COLOR: always
 
 jobs:
-  test:
+  frontend:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+          cache-dependency-path: crates/proxy/admin-ui/package-lock.json
+      - name: Install dependencies
+        working-directory: crates/proxy/admin-ui
+        run: npm ci
+      - name: TypeScript check
+        working-directory: crates/proxy/admin-ui
+        run: npx tsc --noEmit
+      - name: Lint
+        working-directory: crates/proxy/admin-ui
+        run: npm run lint
+      - name: Build
+        working-directory: crates/proxy/admin-ui
+        run: npm run build
+      - name: Upload dist
+        uses: actions/upload-artifact@v4
+        with:
+          name: admin-ui-dist
+          path: crates/proxy/admin-ui/dist/
+
+  test:
+    needs: frontend
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Download dist
+        uses: actions/download-artifact@v4
+        with:
+          name: admin-ui-dist
+          path: crates/proxy/admin-ui/dist/
       - uses: dtolnay/rust-toolchain@stable
         with:
           components: clippy, rustfmt
@@ -63,6 +96,11 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
+      - name: Download dist
+        uses: actions/download-artifact@v4
+        with:
+          name: admin-ui-dist
+          path: crates/proxy/admin-ui/dist/
       - uses: dtolnay/rust-toolchain@stable
         with:
           targets: ${{ matrix.target }}