feat(app): ask what to study in a studyset with sub-studysets, and say when studying fails

- Studying a studyset that has sub-studysets and cards of its own
  first asks: everything (suggested) or this studyset only. Without
  either, it starts straight away. Alone, the back-first setting
  applies as usual and a turn of it is kept (a study button's focus
  may now leave the side to the settings).
- A session that can't be read says so, with Retry; a rating that
  can't be kept says so below the card, which stays to be rated
  again.

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

@@ -440,6 +440,15 @@
   "cardTileConflict": "Auf zwei Geräten geändert",
 
   "studyClose": "Sitzung schließen",
+  "studyLoadFailed": "Diese Sitzung konnte nicht gelesen werden.",
+  "studyRatingFailed": "Deine Bewertung konnte nicht gespeichert werden. Bewerte die Karte noch einmal.",
+  "studyRetry": "Erneut versuchen",
+  "studyScopeTitle": "Lernumfang",
+  "studyScopeDescription": "Dieses Lernset hat Unter-Lernsets. Wähle, welche Karten wiederholt werden sollen.",
+  "studyScopeEverythingTitle": "Alles",
+  "studyScopeEverythingSubtitle": "Dieses Lernset und alle Unter-Lernsets",
+  "studyScopeSelfTitle": "Nur dieses Lernset",
+  "studyScopeSelfSubtitle": "Karten in Unter-Lernsets überspringen",
   "studyBackFirstOn": "Rückseite der Karten wird zuerst gezeigt: tippe zum Zurückschalten",
   "studyBackFirstOff": "Rückseite der Karten zuerst zeigen",
   "studyShowAnswer": "Antwort anzeigen",