feat: android webview app + APK build script + CI artifact

Android app (WebView wrapper):
- Wraps the public household sharing URL in a native Android app
- Swipe-to-refresh, error handling, back navigation, settings dialog
- User enters their household's public URL on first launch
- Material Design with toolbar, progress indicator

Build tooling:
- build-apk.sh: one-command APK build (auto-installs Java, SDK, Gradle)
- .gitignore updated for Android build artifacts

CI (.gitea/workflows/ci.yml):
- New 'build-android' job (needs test-and-package)
- Installs Android SDK + Java 17, builds debug APK
- Uploads APK as 'meal-tracker-apk' artifact

.gitignore

@@ -2,3 +2,10 @@
 meals.db
 __pycache__/
 .pytest_cache/
+
+# Android
+android/.gradle/
+android/.gradle-home/
+android/local.properties
+android/app/build/
+*.apk