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/settings_test.dart

@@ -2,6 +2,7 @@ import 'package:encrypter/encrypter/xor.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:frontend/settings/settings.dart';
+import 'package:frontend/utils/pref_utils.dart';
 
 import '../helpers/backend_test_harness.dart';
 import '../helpers/pump_app.dart';
@@ -24,24 +25,26 @@ void main() {
     expect(find.text('Catppuccin Mocha'), findsOneWidget);
   });
 
-  testWidgets('validates an empty base URL when testing the connection',
-      (tester) async {
+  testWidgets('validates an empty base URL when testing the connection', (
+    tester,
+  ) async {
     await pumpScreen(tester, const Settings());
 
     await tester.enterText(find.byType(TextFormField).first, '');
-    await tester.tap(find.widgetWithText(ElevatedButton, 'Test'));
+    await tester.tap(find.widgetWithText(FilledButton, 'Test'));
     await tester.pump();
 
     expect(find.text('The URL cannot be empty'), findsOneWidget);
   });
 
-  testWidgets('disables Save once the connection details are edited',
-      (tester) async {
+  testWidgets('disables Save once the connection details are edited', (
+    tester,
+  ) async {
     await pumpScreen(tester, const Settings());
-    final saveButton = find.widgetWithText(ElevatedButton, 'Save');
+    final saveButton = find.widgetWithText(FilledButton, 'Save');
 
     expect(
-      tester.widget<ElevatedButton>(saveButton).onPressed,
+      tester.widget<FilledButton>(saveButton).onPressed,
       isNotNull,
       reason: 'enabled for the unmodified, prefilled form',
     );
@@ -53,19 +56,56 @@ void main() {
     await tester.pump();
 
     expect(
-      tester.widget<ElevatedButton>(saveButton).onPressed,
+      tester.widget<FilledButton>(saveButton).onPressed,
       isNull,
       reason: 'disabled until the new connection is tested',
     );
   });
 
-  testWidgets('saving the unmodified form shows a success message',
-      (tester) async {
+  testWidgets('saving the unmodified form shows a success message', (
+    tester,
+  ) async {
     await pumpScreen(tester, const Settings());
 
-    await tester.tap(find.widgetWithText(ElevatedButton, 'Save'));
+    await tester.tap(find.widgetWithText(FilledButton, 'Save'));
     await pumpUntilFound(tester, find.text('Settings saved successfully'));
 
     expect(find.text('Settings saved successfully'), findsOneWidget);
   });
+
+  testWidgets('shows the welcome intro when no server is configured', (
+    tester,
+  ) async {
+    await PrefUtil.setValue('base_url', '');
+    await pumpScreen(tester, const Settings());
+
+    expect(find.textContaining('Welcome to OOTT'), findsOneWidget);
+  });
+
+  testWidgets('hides the welcome intro once a server is configured', (
+    tester,
+  ) async {
+    await PrefUtil.setValue('base_url', 'http://my.server/api');
+    await pumpScreen(tester, const Settings());
+
+    expect(find.textContaining('Welcome to OOTT'), findsNothing);
+  });
+
+  testWidgets('explains why Save is disabled after editing the connection', (
+    tester,
+  ) async {
+    await pumpScreen(tester, const Settings());
+
+    // Editing the connection requires re-testing before saving.
+    await tester.enterText(
+      find.byType(TextFormField).first,
+      'http://changed/api',
+    );
+    await tester.pump();
+
+    expect(
+      find.textContaining('Test the connection before saving'),
+      findsOneWidget,
+    );
+  });
 }