fix: reuse frontend build artifact in e2e instead of rebuilding

The e2e job was running a full Vite build on the same runner as the
backend + Postgres, causing OOM. Now:
- Frontend job uploads dist/ as an artifact
- E2E job depends on frontend, downloads the artifact
- Playwright webServer skips build in CI, just runs preview
- Locally still builds as before

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -95,7 +95,15 @@ jobs:
       - name: Build
         run: cd frontend && NODE_OPTIONS="--max-old-space-size=4096" npm run build
 
+      - name: Upload build artifact
+        uses: actions/upload-artifact@v5
+        with:
+          name: frontend-dist
+          path: frontend/dist
+          retention-days: 1
+
   e2e:
+    needs: [frontend]
     runs-on: ubuntu-latest
 
     services:
@@ -147,6 +155,12 @@ jobs:
       - name: Install frontend dependencies
         run: cd frontend && npm ci
 
+      - name: Download frontend build
+        uses: actions/download-artifact@v5
+        with:
+          name: frontend-dist
+          path: frontend/dist
+
       - name: Install Playwright browser
         run: cd frontend && npx playwright install --with-deps chromium