feat(app,ui): sync waits on mobile data and asks first

The developer's request of 10 Nov 2026, decided in ADR 0042: on mobile
data every app holds its sync back, rows and files alike, and asks.

halkyon_app becomes an Android plugin, its first native code, registered
in every app by Flutter: HalkyonAppPlugin.java says which network the
device is on and whether it is metered as Android has it (no
NET_CAPABILITY_NOT_METERED: cellular, a phone's hotspot, Wi-Fi marked
metered). Java, so it applies no Kotlin Gradle Plugin.

MobileDataGate, one per device, holds the setting (Ask first, Sync
without files, Sync everything; kept in <app support>/mobile_data.json),
the network, and the person's answer, which holds until the device moves
to another network or off a metered one. SyncService asks it before each
sync: held back, nothing goes, the status counts what waits and asks; an
answer or Wi-Fi syncs at once, and syncing now while held back is Sync
now. Each app's frame shows the banner above every tab, in halkyon_ui's
new NavigationFrame.banner slot (On mobile data · 3 changes waiting, with
Not now, Without files and Sync now); the Sync card in Settings chooses
the setting on a phone and says when files wait for Wi-Fi. Learn's files
neither go up nor come down while held back.

Whats-New: On mobile data the app no longer syncs by itself: a banner asks first, and Settings › Sync chooses whether to ask, sync without files, or sync everything.
Whats-New-De: Mit mobilen Daten synchronisiert die App nicht mehr von selbst: Ein Hinweis fragt zuerst, und unter Einstellungen › Synchronisierung wählst du, ob sie fragt, ohne Dateien oder alles synchronisiert.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/lib/l10n/halkyon_app_localizations.dart

@@ -3240,6 +3240,60 @@ abstract class HalkyonAppLocalizations {
   /// In en, this message translates to:
   /// **'Device language'**
   String get lookLanguageDevice;
+
+  /// The Sync card's state while sync waits for the person's answer or Wi-Fi (Halkyon's ADR 0042).
+  ///
+  /// In en, this message translates to:
+  /// **'Held back on mobile data'**
+  String get syncStateHeldBack;
+
+  /// The Sync card's line while rows sync on mobile data and files wait (ADR 0042).
+  ///
+  /// In en, this message translates to:
+  /// **'Files wait for Wi-Fi'**
+  String get syncFilesHeld;
+
+  /// The Sync card's setting: what the device syncs on a metered network unasked (ADR 0042, D5).
+  ///
+  /// In en, this message translates to:
+  /// **'On mobile data'**
+  String get mobileDataTitle;
+
+  /// A choice of mobileDataTitle: sync waits until the person answers the banner.
+  ///
+  /// In en, this message translates to:
+  /// **'Ask first'**
+  String get mobileDataAsk;
+
+  /// A choice of mobileDataTitle: rows sync, files wait for Wi-Fi.
+  ///
+  /// In en, this message translates to:
+  /// **'Sync without files'**
+  String get mobileDataWithoutFiles;
+
+  /// A choice of mobileDataTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'Sync everything'**
+  String get mobileDataEverything;
+
+  /// The banner above every tab while sync is held back on mobile data (ADR 0042, D3); count is the changes made here not synced yet.
+  ///
+  /// In en, this message translates to:
+  /// **'On mobile data · {count, plural, =0{sync paused} =1{1 change waiting} other{{count} changes waiting}}'**
+  String mobileDataBanner(int count);
+
+  /// The banner's action syncing the rows alone, files waiting for Wi-Fi.
+  ///
+  /// In en, this message translates to:
+  /// **'Without files'**
+  String get mobileDataBannerWithoutFiles;
+
+  /// The banner's action hiding it until another change waits.
+  ///
+  /// In en, this message translates to:
+  /// **'Not now'**
+  String get mobileDataBannerNotNow;
 }
 
 class _HalkyonAppLocalizationsDelegate