M2-T12: multi-stage Dockerfile (node build -> python runtime) + frontend CI

- Dockerfile: node:22-slim stage runs npm ci + npm run build; python runtime
  stage COPY --from copies dist to /app/frontend/dist (matches SPA_DIST_DIR);
  runtime image has no node
- .dockerignore: exclude frontend/node_modules and frontend/dist from context
- .github/workflows/frontend.yml: npm ci + codegen-sync + lint/typecheck/test/build
- tests/test_deployment.py: skip COPY --from sources in the context-existence
  check; assert the multi-stage frontend build wiring
- verified with a real docker build (image serves SPA, no node at runtime)

.github/workflows/frontend.yml

@@ -0,0 +1,49 @@
+name: frontend
+
+on:
+  push:
+    branches:
+      - "**"
+  pull_request:
+  workflow_dispatch:
+
+jobs:
+  frontend:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v4
+
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+          cache: npm
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install dependencies
+        working-directory: frontend
+        run: npm ci
+
+      - name: Check codegen is in sync
+        working-directory: frontend
+        run: |
+          npm run codegen
+          git diff --exit-code src/api/schema.d.ts
+
+      - name: Lint
+        working-directory: frontend
+        run: npm run lint
+
+      - name: Type-check
+        working-directory: frontend
+        run: npm run typecheck
+
+      - name: Test
+        working-directory: frontend
+        run: npm run test
+
+      - name: Build
+        working-directory: frontend
+        run: npm run build