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/pubspec.yaml

@@ -88,6 +88,15 @@ dev_dependencies:
 flutter:
   # Generates HalkyonAppLocalizations from lib/l10n (see l10n.yaml).
   generate: true
+  # Android's half of what every app needs from the system, registered in each
+  # app by Flutter itself (decided 10 Nov 2026, Halkyon's ADR 0042): whether
+  # the network is metered (AndroidNetworkWatch). In Java, so it applies no
+  # Kotlin Gradle Plugin, which AGP 9's built-in Kotlin refuses.
+  plugin:
+    platforms:
+      android:
+        package: app.halkyon.halkyon_app
+        pluginClass: HalkyonAppPlugin
   # Material's icon font, which serverpod_auth_idp_flutter's widgets ask for
   # (it sets this itself): without it every test run here warns, and its
   # widgets' icons would draw as boxes in them (each app sets it for itself).