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/learn/halkyon_learn_flutter/lib/src/core/asset/data/asset_repository.dart

@@ -37,6 +37,10 @@ class AssetRepository(
   final AssetFileCache _files,
   final AssetSealing _sealing, {
   Stream<void> uploadTriggers = const Stream.empty(),
+
+  /// Whether files may go up and come down now: not while sync holds them
+  /// back on mobile data (Halkyon's ADR 0042); always where not said.
+  final bool Function() _filesAllowed = _always,
 }) implements TreeFiles {
   this {
     _triggers = uploadTriggers.listen((_) {
@@ -321,6 +325,10 @@ class AssetRepository(
   }
 
   Future<Uint8List?> _download(String id, String hash) async {
+    if (!_filesAllowed()) {
+      // As without a connection: what this device lacks waits.
+      return null;
+    }
     try {
       final bytes = await _server.download(id);
       if (bytes == null || sha256.convert(bytes).toString() != hash) {
@@ -339,7 +347,7 @@ class AssetRepository(
   Future<void> uploadPending() => _uploads.run();
 
   Future<void> _uploadAll() async {
-    if (_stopped) {
+    if (_stopped || !_filesAllowed()) {
       return;
     }
     for (final hash in await _files.pendingUploads()) {
@@ -425,3 +433,5 @@ class AssetRepository(
     await _uploads.idle;
   }
 }
+
+bool _always() => true;

apps/learn/halkyon_learn_flutter/lib/src/core/dependencies/dependencies.dart

@@ -356,6 +356,7 @@ class Dependencies({
             await folders.openOnly(person.authUserId),
             accountKeys: () => accountKeys.read(person.authUserId),
             pdfEngine: pdfEngine,
+            mobileData: device.mobileData,
             onClosed: closed.complete,
           );
         } on Object {
@@ -408,6 +409,10 @@ class DeviceParts({
 
   /// The device's way to the person's Jellyfin server.
   required final JellyfinRepository jellyfin,
+
+  /// 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<DeviceParts> create({required Instance builtIn}) async {
@@ -426,6 +431,9 @@ class DeviceParts({
       deviceSettingsStore: FileDeviceSettingsStore(
         File(p.join(support, 'device_settings.json')),
       ),
+      mobileData: await MobileDataGate.forDevice(
+        File(p.join(support, 'mobile_data.json')),
+      ),
       jellyfin: JellyfinRepository(
         (device) => HttpJellyfinGateway(http.Client(), device: device),
         SecureJellyfinStore(),
@@ -452,6 +460,7 @@ Future<PersonDependencies> _openPersonData(
   Directory folder, {
   required Future<UnlockedAccountKeys?> Function() accountKeys,
   required PdfEngine pdfEngine,
+  required MobileDataGate? mobileData,
   required void Function() onClosed,
 }) async {
   final db = await client.createSyncSession(
@@ -473,6 +482,7 @@ Future<PersonDependencies> _openPersonData(
     ),
     pendingChanges: kept,
     syncedBefore: (await kept.read()).synced,
+    mobileData: mobileData,
   )..start();
   // The person's keys, with which private studysets, their tags and their
   // settings are sealed and opened (ADR 0019).
@@ -535,6 +545,7 @@ Future<PersonDependencies> _openPersonData(
     DirectoryAssetFileCache(Directory(p.join(folder.path, 'files'))),
     privacy,
     uploadTriggers: sync.synced,
+    filesAllowed: () => sync.filesAllowed,
   );
   // A tree made private or not, or moved, takes its files along (ADR 0019);
   // what a run left, or a device that hadn't heard wrote, is sealed as the

apps/learn/halkyon_learn_flutter/lib/src/core/router/app_shell.dart

@@ -48,6 +48,8 @@ class const AppShell({
     // After an update, what's new shows once.
     return WhatsNewOnUpdate(
       child: NavigationFrame(
+        // Sync held back on mobile data asks above every tab (ADR 0042).
+        banner: mobileDataBannerOf(context),
         appName: l10n.appTitle,
         // The icon has a white background, which the frame's rounded tile
         // keeps reading on light and dark rails alike.

apps/learn/halkyon_learn_flutter/test/core/asset/data/asset_repository_test.dart

@@ -18,6 +18,9 @@ void main() {
   late MemoryAssetFileCache cache;
   late StreamController<void> triggers;
   late AssetRepository repository;
+
+  /// Whether files may move now: not on mobile data held back (ADR 0042).
+  late bool filesAllowed;
   final image = Uint8List.fromList(utf8.encode('an image'));
   final imageHash = sha256Of(image);
 
@@ -26,12 +29,14 @@ void main() {
     server = FakeAssetServerGateway();
     cache = MemoryAssetFileCache();
     triggers = StreamController<void>.broadcast();
+    filesAllowed = true;
     repository = AssetRepository(
       rows,
       server,
       cache,
       FakeAssetSealing(),
       uploadTriggers: triggers.stream,
+      filesAllowed: () => filesAllowed,
     );
   });
   tearDown(() async {
@@ -159,6 +164,22 @@ void main() {
       expect(cache.pending, isEmpty);
     });
 
+    test('waits while files are held back on mobile data, and goes once '
+        'they may (ADR 0042)', () async {
+      await addImage();
+      filesAllowed = false;
+
+      triggers.add(null);
+      await pumpEventQueue();
+      expect(server.started, isEmpty);
+      expect(cache.pending, {imageHash});
+
+      filesAllowed = true;
+      triggers.add(null);
+      await pumpEventQueue();
+      expect(server.files, {imageHash: image});
+    });
+
     test('sends nothing the server has, and stops waiting', () async {
       server.files[imageHash] = image;
       await addImage();
@@ -291,6 +312,20 @@ void main() {
       expect(server.downloads, ['remote-image']);
     });
 
+    test('downloads nothing while files are held back on mobile data; one '
+        'this device has is served (ADR 0042)', () async {
+      imageFromElsewhere();
+      filesAllowed = false;
+
+      expect(await repository.fileOf('remote-image'), isNull);
+      expect(server.downloads, isEmpty);
+
+      filesAllowed = true;
+      expect((await repository.fileOf('remote-image'))?.bytes, image);
+      filesAllowed = false;
+      expect((await repository.fileOf('remote-image'))?.bytes, image);
+    });
+
     test('downloads a file once when it is asked for twice at once', () async {
       imageFromElsewhere();