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/helpers/fixtures.dart

@@ -74,6 +74,14 @@ Map<String, dynamic> notificationJson({
   'mac_address': macAddress,
 };
 
+/// Wraps a page of items in the paged-list response shape the list endpoints
+/// return: `{items: [...], total_count: N}`. [totalCount] defaults to the page
+/// length, so single-page stubs stay terse; pass it to simulate more pages.
+Map<String, dynamic> pagedListJson(
+  List<Map<String, dynamic>> items, {
+  int? totalCount,
+}) => {'items': items, 'total_count': totalCount ?? items.length};
+
 /// Shape shared by the ARP and SNMP scanners (run-on-interval scanners).
 Map<String, dynamic> intervalScannerJson({
   bool isRunning = false,