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_de.arb

@@ -1778,8 +1778,7 @@
   "assistantHistoryExplained": "Karte erklärt",
   "assistantHistoryChecked": "Auf Fehler geprüft",
   "assistantHistoryDelete": "Unterhaltung löschen",
-  "assistantHistoryDeleteMessage": "„{title}“ verschwindet von allen deinen Geräten.",
-  "assistantHistoryDeleteConfirm": "Löschen",
+  "assistantHistoryDeleted": "Unterhaltung gelöscht",
   "assistantGone": "Ein nicht mehr angebotener Assistent",
   "assistantGoneNote": "Dieser Assistent wird dir hier nicht mehr angeboten, also lässt sich die Unterhaltung nicht fortsetzen.",
   "assistantNotKept": "Nicht gespeichert: Dieses Gerät kann Unterhaltungen nicht verschlüsseln, also ist diese weg, sobald du sie schließt.",

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.",

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations.dart

@@ -10497,17 +10497,11 @@ abstract class AppLocalizations {
   /// **'Delete conversation'**
   String get assistantHistoryDelete;
 
-  /// No description provided for @assistantHistoryDeleteMessage.
+  /// No description provided for @assistantHistoryDeleted.
   ///
   /// In en, this message translates to:
-  /// **'“{title}” goes from all your devices.'**
-  String assistantHistoryDeleteMessage(String title);
-
-  /// No description provided for @assistantHistoryDeleteConfirm.
-  ///
-  /// In en, this message translates to:
-  /// **'Delete'**
-  String get assistantHistoryDeleteConfirm;
+  /// **'Conversation deleted'**
+  String get assistantHistoryDeleted;
 
   /// No description provided for @assistantGone.
   ///

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations_de.dart

@@ -6634,12 +6634,7 @@ class AppLocalizationsDe extends AppLocalizations {
   String get assistantHistoryDelete => 'Unterhaltung löschen';
 
   @override
-  String assistantHistoryDeleteMessage(String title) {
-    return '„$title“ verschwindet von allen deinen Geräten.';
-  }
-
-  @override
-  String get assistantHistoryDeleteConfirm => 'Löschen';
+  String get assistantHistoryDeleted => 'Unterhaltung gelöscht';
 
   @override
   String get assistantGone => 'Ein nicht mehr angebotener Assistent';

apps/learn/halkyon_learn_flutter/lib/l10n/app_localizations_en.dart

@@ -6574,12 +6574,7 @@ class AppLocalizationsEn extends AppLocalizations {
   String get assistantHistoryDelete => 'Delete conversation';
 
   @override
-  String assistantHistoryDeleteMessage(String title) {
-    return '“$title” goes from all your devices.';
-  }
-
-  @override
-  String get assistantHistoryDeleteConfirm => 'Delete';
+  String get assistantHistoryDeleted => 'Conversation deleted';
 
   @override
   String get assistantGone => 'An assistant no longer offered';