perf(learn): Delete now leaves Recently deleted at once

Deleting a studyset for good blanks and deletes every card, note, path
and PDF of its subtree before the studyset's row goes, which took about
3 s while the studyset stayed in Recently deleted. Now the repository's
Recently deleted leaves it out at once (StudysetRepository.leaving and
stay, applied to the value last read without reading again), the
deletion runs in the background, still one at a time in
StudysetDeletions' queue, and where it fails the studyset shows there
again, read anew with what is left of it, and a snack bar says so.

Whats-New: Delete now in Recently deleted takes a studyset away at once.
Whats-New-De: „Jetzt löschen“ unter „Zuletzt gelöscht“ entfernt ein Lernset sofort.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

apps/learn/halkyon_learn_flutter/lib/l10n/app_de.arb

@@ -36,6 +36,8 @@
   "libraryDeleteForGoodMessage": "„{title}“ wird samt allem, was darin ist, und allen zugehörigen Karten auf all deinen Geräten gelöscht. Das lässt sich nicht rückgängig machen.",
   "@libraryDeleteForGoodMessage": {"placeholders": {"title": {"type": "String"}}},
   "libraryDeleteForGoodConfirm": "Endgültig löschen",
+  "libraryDeleteForGoodFailed": "„{title}“ ließ sich nicht löschen. Es ist wieder unter „Zuletzt gelöscht“.",
+  "@libraryDeleteForGoodFailed": {"placeholders": {"title": {"type": "String"}}},
 
   "studysetSheetCreateTitle": "Lernset erstellen",
   "studysetSheetEditTitle": "Lernset bearbeiten",