feat(linux): suggest the top bar's extension instead of a notification

Decided 2026-10-07: where GNOME's top bar shows no icons (the
AppIndicator extension missing or off), the timer shows in the app
alone, and no notification stands in for it. The Pomodoro card says
that the timer can count down in the top bar, with a button that asks
GNOME Shell to switch the extension on where it is installed or to
install it (GNOME's own dialog), or opens its page on
extensions.gnome.org where Shell can't be asked; until the person says
not to suggest it again. An app can't install or switch on a Shell
extension by itself. The alert at a phase's end stays.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -3090,6 +3090,16 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get pathImportTitleNotText => 'a title must be text.';
+
+  @override
+  String get pomodoroTopBarHint =>
+      'The timer can count down in GNOME\'s top bar, with its buttons, once the AppIndicator extension is on.';
+
+  @override
+  String get pomodoroTopBarSetUp => 'Set up';
+
+  @override
+  String get pomodoroTopBarDismiss => 'Don\'t suggest again';
 }
 
 /// The translations for English, as used in the United Kingdom (`en_GB`).