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_en.arb

@@ -487,6 +487,15 @@
   "cardTileConflict": "Changed on two devices",
 
   "studyClose": "Close session",
+  "studyLoadFailed": "Couldn't read this session.",
+  "studyRatingFailed": "Couldn't keep your rating. Rate the card again.",
+  "studyRetry": "Retry",
+  "studyScopeTitle": "Study scope",
+  "studyScopeDescription": "This studyset has sub-studysets. Pick which cards to review.",
+  "studyScopeEverythingTitle": "Everything",
+  "studyScopeEverythingSubtitle": "This studyset and all sub-studysets",
+  "studyScopeSelfTitle": "This studyset only",
+  "studyScopeSelfSubtitle": "Skip cards in sub-studysets",
   "studyBackFirstOn": "Showing the back of cards first: tap to revert",
   "studyBackFirstOff": "Show the back of cards first",
   "studyShowAnswer": "Show answer",