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/todo/halkyon_todo_flutter/lib/src/app/todo_dependencies.dart

@@ -374,6 +374,7 @@ class TodoDependencies({
             locationAccess: device.locationAccess,
             paneWidths: device.paneWidths,
             completionFeedback: device.completionFeedback,
+            mobileData: device.mobileData,
             onClosed: closed.complete,
           );
         } on Object {
@@ -416,6 +417,7 @@ Future<TodoPerson> _openPersonData(
   required LocationReminderAccess locationAccess,
   required PaneWidths paneWidths,
   required CompletionFeedback completionFeedback,
+  required MobileDataGate? mobileData,
   required void Function() onClosed,
 }) async {
   final db = await client.createSyncSession(
@@ -437,6 +439,7 @@ Future<TodoPerson> _openPersonData(
     ),
     pendingChanges: kept,
     syncedBefore: (await kept.read()).synced,
+    mobileData: mobileData,
   )..start();
   // The person's key, with which everything of theirs is sealed (D5); read
   // again whenever the projects are, as a sync may have brought it.
@@ -537,6 +540,10 @@ class TodoDevice({
 
   /// What a task done feels like, the sound turned off or not.
   required final CompletionFeedback completionFeedback,
+
+  /// What sync may do on the network the device is on (Halkyon's ADR
+  /// 0042): every person's on this device.
+  final MobileDataGate? mobileData,
 }) {
   /// Whether the device may show reminders, asked once an app start,
   /// whoever is signed in (D31).
@@ -553,6 +560,9 @@ class TodoDevice({
     final support = (await getApplicationSupportDirectory()).path;
     return TodoDevice(
       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/todo/halkyon_todo_flutter/lib/src/tabs/todo_shell.dart

@@ -52,6 +52,8 @@ class _TodoShellState() extends State<TodoShell> {
         listenable: Listenable.merge([_selection, _hold]),
         builder: (context, child) => WhatsNewOnUpdate(
           child: 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>(