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/empty_state_test.dart

@@ -0,0 +1,48 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/widgets/empty_state.dart';
+
+import '../helpers/backend_test_harness.dart';
+import '../helpers/pump_app.dart';
+
+void main() {
+  setUp(() async {
+    await setUpBackendForTest();
+  });
+
+  testWidgets('renders the icon and message without an action button', (
+    tester,
+  ) async {
+    await pumpScreen(
+      tester,
+      const EmptyState(
+        icon: Icons.devices_other_outlined,
+        message: 'No devices found',
+      ),
+    );
+
+    expect(find.text('No devices found'), findsOneWidget);
+    expect(find.byIcon(Icons.devices_other_outlined), findsOneWidget);
+    expect(find.byType(OutlinedButton), findsNothing);
+  });
+
+  testWidgets('renders an action button and invokes its callback', (
+    tester,
+  ) async {
+    var tapped = false;
+    await pumpScreen(
+      tester,
+      EmptyState(
+        icon: Icons.devices_other_outlined,
+        message: 'No devices found',
+        actionLabel: 'Check scanner status',
+        onAction: () => tapped = true,
+      ),
+    );
+
+    await tester.tap(
+      find.widgetWithText(OutlinedButton, 'Check scanner status'),
+    );
+    expect(tapped, isTrue);
+  });
+}