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/lib/widgets/scanners_status_card.dart

@@ -179,8 +179,8 @@ class _ScannersStatusCardState extends State<ScannersStatusCard>
                 crossAxisAlignment: CrossAxisAlignment.start,
                 children: [
                   Text(
-                    'Status',
-                    style: Theme.of(context).textTheme.titleMedium,
+                    'Scanners',
+                    style: Theme.of(context).textTheme.titleLarge,
                   ),
                   const SizedBox(height: 12),
                   _scannerRow(