Add "go to last page" and page count to notifications and devices lists

The list endpoints now return a total count alongside the page so the
front-end can show how many pages exist and offer a last-page jump.

Backend: add count(is_new) and count_devices(...) (sharing a WHERE-builder
with list_devices so page and count can't drift), wrap both list responses
in {items, total_count} structs, and register them with utoipa.

Front-end: parse the wrapper shape (dropping the fetch-one-extra trick),
add a Last-page button and a responsive "Page X of Y" / "X / Y" label to
the shared PaginationBar, and track the total in both lists. Notifications
re-sync the count on every fetch and decrement it locally on mark-read/
unread removals so the count stays accurate without a re-fetch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/test/widget/home_screen_test.dart

@@ -17,9 +17,10 @@ void main() {
   void stubHomeEndpoints() {
     adapter.onGet(
       '/notifications',
-      (server) => server.reply(200, [
-        notificationJson(id: 1, title: 'New device found'),
-      ]),
+      (server) => server.reply(
+        200,
+        pagedListJson([notificationJson(id: 1, title: 'New device found')]),
+      ),
     );
     adapter.onGet(
       '/devices/summary',
@@ -47,8 +48,9 @@ void main() {
     );
   }
 
-  testWidgets('shows notifications, the device summary and scanner statuses',
-      (tester) async {
+  testWidgets('shows notifications, the device summary and scanner statuses', (
+    tester,
+  ) async {
     stubHomeEndpoints();
 
     // Below the 700px breakpoint the home screen uses its single-column layout,