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/build.gradle

@@ -0,0 +1,38 @@
+plugins {
+    id 'com.android.application'
+}
+
+android {
+    namespace 'com.mealtracker.app'
+    compileSdk 34
+
+    defaultConfig {
+        applicationId "com.mealtracker.app"
+        minSdk 24
+        targetSdk 34
+        versionCode 1
+        versionName "1.0"
+    }
+
+    buildTypes {
+        release {
+            minifyEnabled false
+            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+        }
+        debug {
+            applicationIdSuffix ".debug"
+            versionNameSuffix "-debug"
+        }
+    }
+
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_17
+        targetCompatibility JavaVersion.VERSION_17
+    }
+}
+
+dependencies {
+    implementation 'androidx.appcompat:appcompat:1.6.1'
+    implementation 'androidx.swiperefreshlayout:swiperefreshlayout:1.1.0'
+    implementation 'com.google.android.material:material:1.11.0'
+}