feat(app): deleting a studyset can be undone for a moment

Decided 2026-10-01. Deleting still asks first; once confirmed, the studyset
leaves the library at once and "Studyset deleted" offers Undo for three
seconds. Only when the time to undo is over (four seconds: the message's
three and a moment for a late tap) are its cards and then the studyset
blanked and deleted, since blanking can't be undone (ADR 0002, rule 2). A
deletion still waiting when the library closes (the app quits, the person
changes) is dropped, and the studyset stays.

The library's bloc keeps the waiting deletions; the event that ends the
time to undo is private, so no screen can skip it. AGENTS.md now describes
the library's shared widgets, the sheets (and why a sheet that asks before
discarding can't be swiped away), the order of studysets, and this Undo.

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

@@ -25,8 +25,9 @@
   "librarySearchNoMatches": "No studysets match your search",
   "libraryStudysetArchived": "Studyset archived",
   "libraryStudysetUnarchived": "Studyset unarchived",
+  "libraryStudysetDeleted": "Studyset deleted",
   "libraryDeleteTitle": "Delete studyset?",
-  "libraryDeleteMessage": "It is deleted with its cards, on all your devices.",
+  "libraryDeleteMessage": "It is deleted with its cards, on all your devices. You can undo this for a few seconds.",
   "libraryDeleteConfirm": "Delete",
 
   "studysetSheetCreateTitle": "Create studyset",