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/status/status_screen.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 
+import '../theme/dimens.dart';
 import '../widgets/arp_scanner_card.dart';
 import '../widgets/dhcp_scanner_card.dart';
 import '../widgets/mdns_scanner_card.dart';
@@ -15,16 +16,14 @@ class StatusScreen extends StatelessWidget {
       child: Column(
         crossAxisAlignment: CrossAxisAlignment.start,
         children: [
-          Text('Status', style: Theme.of(context).textTheme.headlineMedium),
-          const SizedBox(height: 20),
           const ArpScannerCard(),
-          const SizedBox(height: 8),
+          const SizedBox(height: Insets.sm),
           const MdnsScannerCard(),
-          const SizedBox(height: 8),
+          const SizedBox(height: Insets.sm),
           const SsdpScannerCard(),
-          const SizedBox(height: 8),
+          const SizedBox(height: Insets.sm),
           const DhcpScannerCard(),
-          const SizedBox(height: 8),
+          const SizedBox(height: Insets.sm),
           const SnmpScannerCard(),
         ],
       ),