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/empty_state.dart

@@ -0,0 +1,51 @@
+import 'package:flutter/material.dart';
+
+import '../theme/dimens.dart';
+
+/// A centered placeholder for "nothing here yet" situations: an icon, a short
+/// message and an optional call-to-action button. Keeps empty screens
+/// consistent and gives the user a next step instead of a bare line of text.
+class EmptyState extends StatelessWidget {
+  const EmptyState({
+    required this.icon,
+    required this.message,
+    this.actionLabel,
+    this.onAction,
+    super.key,
+  });
+
+  final IconData icon;
+  final String message;
+  final String? actionLabel;
+  final VoidCallback? onAction;
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    final hasAction = actionLabel != null && onAction != null;
+
+    return Center(
+      child: Padding(
+        padding: const EdgeInsets.all(Insets.xxl),
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            Icon(icon, size: 48, color: theme.colorScheme.onSurfaceVariant),
+            const SizedBox(height: Insets.md),
+            Text(
+              message,
+              textAlign: TextAlign.center,
+              style: theme.textTheme.bodyLarge?.copyWith(
+                color: theme.colorScheme.onSurfaceVariant,
+              ),
+            ),
+            if (hasAction) ...[
+              const SizedBox(height: Insets.lg),
+              OutlinedButton(onPressed: onAction, child: Text(actionLabel!)),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}