One app menu style for every picker; links go Through me by default

- AppSelect / AppMenuButton (widgets/app_menu.dart): glass menu anchored
  to its button, check on the current item, keyboard and Esc; replaces
  the Material dropdowns in Settings, player settings, Jellyfin and the
  member menu. Material menus elsewhere get the same surface by theme.
- Link page: Through me is the main choice (you can't know which sites
  need cookies); Everyone / Server fetches it sit under "Other ways".

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/app.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'src/screens/home/ui.dart';
 import 'src/shell/app_shell.dart';
 import 'src/widgets/app_keys.dart';
+import 'src/widgets/app_menu.dart';
 
 class WatchTogetherApp extends StatelessWidget {
   const WatchTogetherApp({super.key});
@@ -11,7 +12,9 @@ class WatchTogetherApp extends StatelessWidget {
   Widget build(BuildContext context) {
     return MaterialApp(
       title: appName,
-      theme: ThemeData.dark(),
+      // Material's own menus (the player's track menus, sort) wear the
+      // app menu's surface.
+      theme: AppMenuStyle.theme(ThemeData.dark()),
       shortcuts: appShortcuts,
       home: const AppKeys(child: AppShell()),
     );