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_de.arb

@@ -859,5 +859,8 @@
   "pathImportNotText": "„{detail}“ muss Text sein.",
   "pathImportTooFew": "es braucht mindestens zwei „{detail}“.",
   "pathImportMalformedNumber": "eine fehlerhafte Zahl.",
-  "pathImportTitleNotText": "ein Titel muss Text sein."
+  "pathImportTitleNotText": "ein Titel muss Text sein.",
+  "pomodoroTopBarHint": "Der Timer kann in der oberen Leiste von GNOME herunterzählen, mit seinen Knöpfen, sobald die Erweiterung „AppIndicator“ an ist.",
+  "pomodoroTopBarSetUp": "Einrichten",
+  "pomodoroTopBarDismiss": "Nicht mehr vorschlagen"
 }