ci: restore pr-check.yml workflow for dev branch

- Restore backend and frontend PR checks
- Enable Python syntax check and import test for backend
- Enable pnpm build check for frontend
- Fix 'Expected — Waiting for status to be reported' issue

Author: FutureUnreal

.github/workflows/pr-check.yml

@@ -15,76 +15,41 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v5
-
-      - name: Detect backend
-        id: detect
-        run: |
-          if [ -f backend/pyproject.toml ]; then
-            echo "exists=true" >> "$GITHUB_OUTPUT"
-          else
-            echo "exists=false" >> "$GITHUB_OUTPUT"
-          fi
-
-      - name: Skip backend checks (no backend/)
-        if: steps.detect.outputs.exists != 'true'
-        run: echo "No backend/ found; skipping backend checks."
-
       - name: Setup Python
-        if: steps.detect.outputs.exists == 'true'
         uses: actions/setup-python@v5
         with:
           python-version: "3.11"
           cache: "pip"
           cache-dependency-path: backend/pyproject.toml
       - name: Install backend (editable)
-        if: steps.detect.outputs.exists == 'true'
         run: |
           python -m pip install --upgrade pip
           pip install -e backend
       - name: Backend syntax check (compileall)
-        if: steps.detect.outputs.exists == 'true'
         run: python -m compileall -q backend/app
       - name: Backend import smoke test
-        if: steps.detect.outputs.exists == 'true'
         run: python -c "from app.main import app; print('backend app import: ok')"
 
   frontend:
     if: github.event.pull_request.draft == false
     runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: frontend
     steps:
       - uses: actions/checkout@v5
-
-      - name: Detect frontend
-        id: detect
-        run: |
-          if [ -f frontend/pnpm-lock.yaml ] || [ -f frontend/package.json ]; then
-            echo "exists=true" >> "$GITHUB_OUTPUT"
-          else
-            echo "exists=false" >> "$GITHUB_OUTPUT"
-          fi
-
-      - name: Skip frontend checks (no frontend/)
-        if: steps.detect.outputs.exists != 'true'
-        run: echo "No frontend/ found; skipping frontend checks."
-
       - name: Setup Node.js
-        if: steps.detect.outputs.exists == 'true'
         uses: actions/setup-node@v4
         with:
           node-version: "20"
           cache: "pnpm"
           cache-dependency-path: frontend/pnpm-lock.yaml
       - name: Setup pnpm
-        if: steps.detect.outputs.exists == 'true'
         uses: pnpm/action-setup@v4
         with:
           version: "9.12.2"
           run_install: false
       - name: Install dependencies
-        if: steps.detect.outputs.exists == 'true'
-        working-directory: frontend
         run: pnpm install --frozen-lockfile
       - name: Build web app
-        if: steps.detect.outputs.exists == 'true'
-        working-directory: frontend
         run: pnpm --filter @whalewhisper/web build