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>

apps/train/halkyon_train_flutter/lib/src/app/train_dependencies.dart

@@ -373,6 +373,7 @@ class TrainDependencies({
             await folders.openOnly(person.authUserId),
             appVersion: device.appVersion,
             accountKeys: () => accountKeys.read(person.authUserId),
+            mobileData: device.mobileData,
             onClosed: closed.complete,
           );
         } on Object {
@@ -410,6 +411,7 @@ Future<TrainPerson> _openPersonData(
   Directory folder, {
   required String appVersion,
   required Future<UnlockedAccountKeys?> Function() accountKeys,
+  required MobileDataGate? mobileData,
   required void Function() onClosed,
 }) async {
   final db = await client.createSyncSession(
@@ -431,6 +433,7 @@ Future<TrainPerson> _openPersonData(
     ),
     pendingChanges: kept,
     syncedBefore: (await kept.read()).synced,
+    mobileData: mobileData,
   )..start();
   // The person's key, with which everything of theirs is sealed (D15); read
   // again whenever the workouts are, as a sync may have brought it.
@@ -595,6 +598,10 @@ class TrainDevice({
   /// This build's version (before the `+`), which Jellyfin lists Train's
   /// sign-ins by (D47).
   final String appVersion = '',
+
+  /// What sync may do on the network the device is on (Halkyon's ADR
+  /// 0042): every person's on this device.
+  final MobileDataGate? mobileData,
 }) {
   /// Makes them, [builtIn] being the instance the build names.
   static Future<TrainDevice> create({required Instance builtIn}) async {
@@ -602,6 +609,9 @@ class TrainDevice({
     final support = (await getApplicationSupportDirectory()).path;
     return TrainDevice(
       support: support,
+      mobileData: await MobileDataGate.forDevice(
+        File(p.join(support, 'mobile_data.json')),
+      ),
       storage: InstanceStorage(support, builtIn: builtIn),
       appBuild: int.tryParse(package.buildNumber),
       appVersion: package.version,

apps/train/halkyon_train_flutter/lib/src/tabs/train_shell.dart

@@ -49,6 +49,8 @@ class const TrainShell({
           (bloc) => showsRecordingBar(bloc.state),
         );
     final frame = NavigationFrame(
+      // Sync held back on mobile data asks above every tab (ADR 0042).
+      banner: mobileDataBannerOf(context),
       // Open into the sidebar or not, as the device keeps it (decided
       // 10 Nov 2026).
       sidebarOpen: context.select<LookCubit, bool>(