ci: add GitHub Actions build workflow

.github/workflows/build-apk.yml

@@ -0,0 +1,29 @@
+name: Build Debug APK
+
+on:
+  workflow_dispatch:
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          java-version: '17'
+          distribution: 'temurin'
+
+      - name: Setup Android SDK
+        uses: android-actions/setup-android@v3
+
+      - name: Build debug APK
+        run: ./gradlew assembleDebug
+
+      - name: Upload APK
+        uses: actions/upload-artifact@v4
+        with:
+          name: helios-alarm-clock-debug
+          path: app/build/outputs/apk/debug/*.apk
+          retention-days: 7