perf(learn): a conversation deleted goes at once, with an Undo

Deleting a kept conversation asked first and then waited while its
turns were blanked and deleted. It now goes as a card or a note does:
ConversationDeletions, a DelayedDeletions opened with the person's
data, takes it off the history at once (the list leaves out what is
pending), offers Undo for a moment on the sheet (hasSnackBars), and
blanks and deletes it once that is over; a deletion that fails shows
the conversation again, as every DelayedDeletions does. The question
before deleting goes, as Undo replaces it.

Whats-New: A conversation with an assistant is deleted at once, and Undo brings it back.
Whats-New-De: Eine Unterhaltung mit einem Assistenten wird sofort gelöscht, und „Rückgängig“ holt sie zurück.
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_en.arb

@@ -2117,9 +2117,7 @@
   "assistantHistoryChecked": "Checked for mistakes",
   "@assistantHistoryChecked": {"description": "A conversation that checked what was written (D54)."},
   "assistantHistoryDelete": "Delete conversation",
-  "assistantHistoryDeleteMessage": "“{title}” goes from all your devices.",
-  "@assistantHistoryDeleteMessage": {"placeholders": {"title": {"type": "String"}}},
-  "assistantHistoryDeleteConfirm": "Delete",
+  "assistantHistoryDeleted": "Conversation deleted",
   "assistantGone": "An assistant no longer offered",
   "assistantGoneNote": "This assistant is no longer offered to you here, so the conversation can't go on.",
   "assistantNotKept": "Not kept: this device can't encrypt conversations, so this one is gone once you close it.",