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/src/main/res/layout/dialog_settings.xml

@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="wrap_content"
+    android:orientation="vertical"
+    android:padding="16dp">
+
+    <TextView
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:text="@string/url_label"
+        android:textSize="14sp"
+        android:textColor="@android:color/darker_gray"
+        android:paddingBottom="4dp" />
+
+    <EditText
+        android:id="@+id/url_input"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:inputType="textUri"
+        android:hint="https://meals.example.com/public/..."
+        android:padding="12dp"
+        android:background="@android:drawable/edit_text"
+        android:textSize="14sp"
+        android:importantForAutofill="no" />
+
+    <TextView
+        android:id="@+id/url_hint"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"
+        android:textSize="12sp"
+        android:textColor="@android:color/darker_gray"
+        android:paddingTop="8dp" />
+
+</LinearLayout>