Add Gitea Actions workflow to build a release APK on push to main

Checks out, sets up JDK 17 + Flutter (stable) + Android SDK, runs
flutter analyze, then builds and uploads a release APK artifact.
Requires a registered Gitea Actions runner - see the setup notes
given alongside this commit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/build.yml

@@ -0,0 +1,42 @@
+name: Build APK
+
+on:
+  push:
+    branches:
+      - main
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out code
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          distribution: temurin
+          java-version: "17"
+
+      - name: Set up Flutter
+        uses: subosito/flutter-action@v2
+        with:
+          channel: stable
+
+      - name: Set up Android SDK
+        uses: android-actions/setup-android@v3
+
+      - name: Install dependencies
+        run: flutter pub get
+
+      - name: Analyze
+        run: flutter analyze
+
+      - name: Build release APK
+        run: flutter build apk --release
+
+      - name: Upload APK
+        uses: actions/upload-artifact@v4
+        with:
+          name: app-release-apk
+          path: build/app/outputs/flutter-apk/app-release.apk