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/status_screen_test.dart

@@ -43,14 +43,15 @@ void main() {
     );
   }
 
-  testWidgets('renders each scanner card with its resolved status',
-      (tester) async {
+  testWidgets('renders each scanner card with its resolved status', (
+    tester,
+  ) async {
     stubAllScanners();
 
     await pumpScreen(tester, const StatusScreen());
     await pumpUntilFound(tester, find.text('Running'));
 
-    expect(find.text('Status'), findsOneWidget);
+    // The screen title now lives in the shared shell AppBar, not the body.
     expect(find.text('ARP Scanner'), findsOneWidget);
     expect(find.text('mDNS Scanner'), findsOneWidget);
     expect(find.text('Running'), findsWidgets);