Add GitHub Actions CI workflow

Author: colto

.github/workflows/ci.yml

@@ -0,0 +1,52 @@
+name: CI
+
+on:
+  push:
+    branches: ["main"]
+  pull_request:
+    branches: ["main"]
+
+jobs:
+  backend-build:
+    name: Backend Build (Spring Boot)
+    runs-on: ubuntu-latest
+
+    defaults:
+      run:
+        working-directory: backend
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+
+      - name: Set up Java
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: "17"
+
+      - name: Build backend
+        run: mvn -q -DskipTests package
+
+  frontend-build:
+    name: Frontend Build (React)
+    runs-on: ubuntu-latest
+
+    defaults:
+      run:
+        working-directory: frontend
+
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+
+      - name: Set up Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Build frontend
+        run: npm run build