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/test/sync/mobile_data_gate_test.dart

@@ -0,0 +1,149 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:halkyon_app/testing.dart';
+import 'package:path/path.dart' as p;
+
+void main() {
+  late FakeNetworkWatch network;
+  late MobileDataGate gate;
+
+  setUp(() {
+    network = FakeNetworkWatch();
+    gate = MobileDataGate(network: network, offered: true);
+    addTearDown(gate.dispose);
+  });
+
+  test('on Wi-Fi everything goes, whatever is set', () async {
+    await gate.ready;
+    for (final setting in MobileData.values) {
+      await gate.choose(setting);
+      expect(gate.metered, isFalse);
+      expect(gate.allowance, SyncAllowance.everything);
+      expect(gate.filesAllowed, isTrue);
+    }
+  });
+
+  test('on mobile data, asking holds back everything until answered', () async {
+    network.state = FakeNetworkWatch.mobile;
+    await gate.ready;
+
+    expect(gate.metered, isTrue);
+    expect(gate.allowance, SyncAllowance.nothing);
+    expect(gate.filesAllowed, isFalse);
+
+    gate.answer(SyncAllowance.rows);
+    expect(gate.allowance, SyncAllowance.rows, reason: 'without files');
+    expect(gate.filesAllowed, isFalse);
+
+    gate.answer(SyncAllowance.everything);
+    expect(gate.filesAllowed, isTrue);
+  });
+
+  test("set to Without files or Everything, mobile data doesn't ask", () async {
+    network.state = FakeNetworkWatch.mobile;
+    await gate.ready;
+
+    await gate.choose(MobileData.withoutFiles);
+    expect(gate.allowance, SyncAllowance.rows);
+    await gate.choose(MobileData.everything);
+    expect(gate.allowance, SyncAllowance.everything);
+  });
+
+  test('an answer holds until the connection changes: Wi-Fi come and gone, '
+      'or another metered network, asks again', () async {
+    network.state = FakeNetworkWatch.mobile;
+    await gate.ready;
+    var changes = 0;
+    gate
+      ..addListener(() => changes++)
+      ..answer(SyncAllowance.everything);
+    expect(changes, 1);
+
+    network.moveTo(FakeNetworkWatch.mobile);
+    expect(changes, 1, reason: 'the same network again changes nothing');
+    expect(gate.allowance, SyncAllowance.everything);
+
+    network
+      ..moveTo(FakeNetworkWatch.wifi)
+      ..moveTo(FakeNetworkWatch.mobile);
+    expect(gate.allowance, SyncAllowance.nothing);
+
+    gate.answer(SyncAllowance.everything);
+    network.moveTo((id: 'hotspot', metered: true));
+    expect(gate.allowance, SyncAllowance.nothing);
+  });
+
+  test('nothing is answered off a metered network', () async {
+    await gate.ready;
+    gate.answer(SyncAllowance.everything);
+    network.moveTo(FakeNetworkWatch.mobile);
+    expect(gate.allowance, SyncAllowance.nothing);
+  });
+
+  test('a desktop is never metered, and is offered no choice', () async {
+    final desktop = MobileDataGate();
+    addTearDown(desktop.dispose);
+    await desktop.ready;
+    expect(desktop.offered, isFalse);
+    expect(desktop.allowance, SyncAllowance.everything);
+  });
+
+  group('kept on the device', () {
+    late Directory folder;
+
+    setUp(() async {
+      folder = await Directory.systemTemp.createTemp('mobile_data');
+      addTearDown(() => folder.delete(recursive: true));
+    });
+
+    test('the setting stays when the app opens again; an answer goes with '
+        'the app', () async {
+      final file = File(p.join(folder.path, 'mobile_data.json'));
+      network.state = FakeNetworkWatch.mobile;
+      final first = await MobileDataGate.open(
+        file,
+        network: network,
+        offered: true,
+      );
+      addTearDown(first.dispose);
+      await first.ready;
+      await first.choose(MobileData.withoutFiles);
+
+      final again = await MobileDataGate.open(
+        file,
+        network: network,
+        offered: true,
+      );
+      addTearDown(again.dispose);
+      await again.ready;
+      expect(again.mobileData, MobileData.withoutFiles);
+
+      await again.choose(MobileData.ask);
+      again.answer(SyncAllowance.everything);
+      final third = await MobileDataGate.open(
+        file,
+        network: network,
+        offered: true,
+      );
+      addTearDown(third.dispose);
+      await third.ready;
+      expect(third.allowance, SyncAllowance.nothing);
+    });
+
+    test('a file that is no setting asks', () async {
+      final file = File(p.join(folder.path, 'mobile_data.json'));
+      for (final content in ['not json', '[1]', '{"mobileData": "never"}']) {
+        await file.writeAsString(content);
+        final gate = await MobileDataGate.open(
+          file,
+          network: network,
+          offered: true,
+        );
+        addTearDown(gate.dispose);
+        expect(gate.mobileData, MobileData.ask, reason: content);
+      }
+    });
+  });
+}