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/test/home_test.dart

@@ -87,7 +87,7 @@ void main() {
     expect(c.source, MediaKind.jellyfin);
     c.pasteText(' https://example.com/a.mkv ');
     expect(c.source, MediaKind.link);
-    expect(c.selection, const LinkMedia('https://example.com/a.mkv'));
+    expect(c.selection, const LinkMedia('https://example.com/a.mkv', relay: LinkRelay.me));
     c.pasteText('/home/me/b.mkv');
     expect(c.source, MediaKind.file);
     expect(c.selection, const FileMedia('/home/me/b.mkv'));