ci: add Android CI/CD workflow

.github/workflows/android-ci.yml:
- Triggers on push to main and pull requests
- Ubuntu-latest runner with Java 17 (Temurin)
- Gradle cache via gradle/actions/setup-gradle
- Jobs: assembleDebug build + unit tests (./gradlew test)
- Uploads debug APK as workflow artifact (14-day retention)

Closes #14

.github/workflows/android-ci.yml

@@ -0,0 +1,39 @@
+name: Android CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  build:
+    name: Build & Test
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: 17
+
+      - name: Setup Gradle
+        uses: gradle/actions/setup-gradle@v4
+
+      - name: Build debug APK
+        run: ./gradlew assembleDebug
+
+      - name: Run unit tests
+        run: ./gradlew test
+
+      - name: Upload debug APK
+        uses: actions/upload-artifact@v4
+        if: success()
+        with:
+          name: debug-apk
+          path: app/build/outputs/apk/debug/*.apk
+          retention-days: 14