Show a page-level progress bar while paginating lists

Changing pages in the notifications or devices list gave no cue that
the next page was loading. Render an indeterminate progress bar at the
shell level, pinned flush against the bottom of the page body (above
the nav bar on phones, the screen bottom on wide layouts), driven by a
shared paginationLoading notifier the lists set while fetching. The
pagination bar keeps disabling its buttons during the fetch.

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

frontend/test/widget/pagination_bar_test.dart

@@ -0,0 +1,50 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/theme/gruvbox_theme.dart';
+import 'package:frontend/widgets/pagination_bar.dart';
+
+void main() {
+  Widget wrap(Widget child) => MaterialApp(
+    theme: gruvboxDarkTheme,
+    home: Scaffold(body: child),
+  );
+
+  testWidgets('disables every navigation button while loading', (tester) async {
+    var changedTo = -1;
+    await tester.pumpWidget(
+      wrap(
+        PaginationBar(
+          currentPage: 1,
+          hasNextPage: true,
+          isLoading: true,
+          onPageChanged: (page) => changedTo = page,
+        ),
+      ),
+    );
+
+    await tester.tap(find.byTooltip('Next page'));
+    await tester.tap(find.byTooltip('Previous page'));
+    await tester.tap(find.byTooltip('First page'));
+    expect(changedTo, -1);
+  });
+
+  testWidgets('enables navigation when idle', (tester) async {
+    var changedTo = -1;
+    await tester.pumpWidget(
+      wrap(
+        PaginationBar(
+          currentPage: 1,
+          hasNextPage: true,
+          isLoading: false,
+          onPageChanged: (page) => changedTo = page,
+        ),
+      ),
+    );
+
+    await tester.tap(find.byTooltip('Next page'));
+    expect(changedTo, 2);
+
+    await tester.tap(find.byTooltip('Previous page'));
+    expect(changedTo, 0);
+  });
+}