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_ui/lib/src/widget/navigation_frame.dart

@@ -92,6 +92,12 @@ class const NavigationFrame({
   /// while null.
   final Widget? status,
 
+  /// What stands above every tab while it matters (halkyon_app's banner on
+  /// mobile data, Halkyon's ADR 0042): its screens below it without the
+  /// system's top inset, which the banner keeps clear of itself (a
+  /// `SafeArea` without its bottom); none while null.
+  final Widget? banner,
+
   /// Whether the rail is open into the sidebar, from expanded windows on.
   final bool sidebarOpen = false,
 
@@ -224,15 +230,18 @@ class _NavigationFrameState() extends State<NavigationFrame> {
               ),
             ),
           ),
-          child: switch (widget.status) {
-            final status? => Column(
-              children: [
-                Expanded(child: widget.child),
-                Padding(padding: const EdgeInsets.all(12), child: status),
-              ],
-            ),
-            null => widget.child,
-          },
+          child: _underBanner(
+            context,
+            switch (widget.status) {
+              final status? => Column(
+                children: [
+                  Expanded(child: widget.child),
+                  Padding(padding: const EdgeInsets.all(12), child: status),
+                ],
+              ),
+              null => widget.child,
+            },
+          ),
         ),
       );
     }
@@ -269,6 +278,24 @@ class _NavigationFrameState() extends State<NavigationFrame> {
     );
   }
 
+  /// [screens] below the banner, where there is one.
+  Widget _underBanner(BuildContext context, Widget screens) =>
+      switch (widget.banner) {
+        final banner? => Column(
+          children: [
+            banner,
+            Expanded(
+              child: MediaQuery.removePadding(
+                context: context,
+                removeTop: true,
+                child: screens,
+              ),
+            ),
+          ],
+        ),
+        null => screens,
+      };
+
   /// The frame below medium windows, its bar in [colour], the screens on
   /// [background].
   Widget _phone(
@@ -314,7 +341,10 @@ class _NavigationFrameState() extends State<NavigationFrame> {
         showsBar: !widget.hidesNavigationBar,
         child: _FrameFadeColour(
           background: background,
-          child: ScaffoldMessenger(key: _phoneMessenger, child: widget.child),
+          child: ScaffoldMessenger(
+            key: _phoneMessenger,
+            child: _underBanner(context, widget.child),
+          ),
         ),
       ),
       // Without the bar, a strip as high as the system's own bar, in the