ci: add GitHub Actions for frontend build and backend syntax check

Author: ergon73

.github/workflows/ci.yml

@@ -0,0 +1,51 @@
+name: CI
+
+on:
+  push:
+    branches: [ main, dev ]
+  pull_request:
+    branches: [ main, dev ]
+
+jobs:
+  frontend-build:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: frontend
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Use Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: 'npm'
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install deps
+        run: npm ci
+
+      - name: Build (REACT_APP_API_URL dummy)
+        run: REACT_APP_API_URL=http://localhost:5000 npm run build
+
+  backend-validate:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - name: Install backend deps
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r backend/requirements.txt
+
+      - name: Basic syntax check
+        run: python -m compileall -q backend
+
+