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

android/app/proguard-rules.pro

@@ -0,0 +1,12 @@
+# Add project specific ProGuard rules here.
+# By default, the flags in this file are appended to flags specified
+# in the SDK tools.
+
+# WebView - keep JavaScript interface methods
+-keepclassmembers class * {
+    @android.webkit.JavascriptInterface <methods>;
+}
+
+# Keep material components
+-dontwarn com.google.android.material.**
+-keep class com.google.android.material.** { *; }