Polish frontend: design tokens, theming, AppBar titles, empty/loading states

Frontend usability and aesthetics pass across web and mobile:

- Add shared layout tokens (theme/dimens.dart: Insets + Breakpoints) and
  replace magic-number spacing and per-file breakpoint consts.
- Route both themes through buildAppTheme (theme/theme_builder.dart): explicit
  useMaterial3 and a shared branded textTheme (Barlow Condensed for
  display/headline/title styles); logo now reads its style from the theme.
- Move screen titles into the shared AppBar (route-derived) and drop the
  redundant in-body headers; upgrade Settings buttons to M3 FilledButton.
- Add reusable EmptyState and skeleton loaders; Devices empty state links to
  scanner status, notifications get filter-aware messages.
- Add a first-run welcome intro and Save-disabled helper text in Settings.
- Make device Filter/Sort adaptive: bottom sheet on phones, dialog on wide.
- Collapse the device-list filter chips and Sort/Filter buttons into one row;
  rename the home Scanners card title and align its style.

Tests: add EmptyState/skeleton widget tests and Settings first-run cases;
update tests for the FilledButton swap and relocated titles. 89 passing,
dart analyze clean.

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

frontend/test/widget/skeleton_test.dart

@@ -0,0 +1,24 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/widgets/skeleton.dart';
+
+import '../helpers/backend_test_harness.dart';
+import '../helpers/pump_app.dart';
+
+void main() {
+  setUp(() async {
+    await setUpBackendForTest();
+  });
+
+  testWidgets('ListSkeleton renders three placeholder blocks per row', (
+    tester,
+  ) async {
+    await pumpScreen(tester, const ListSkeleton(rows: 2));
+    await tester.pump();
+
+    // Each row is an avatar block plus two text-line blocks.
+    expect(find.byType(Skeleton), findsNWidgets(6));
+
+    // Unmount so the pulsing animation controllers are disposed cleanly.
+    await tearDownTree(tester);
+  });
+}