simplified code across the app and consolidated duplicated helpers

lib/push/chat_thread_store.dart

@@ -6,13 +6,20 @@ import 'package:localstore/localstore.dart';
 /// notification, small enough to stay cheap in the background isolate.
 const int kChatThreadCap = 15;
 
-/// Canonical per-chat notification tag (kept in sync with
-/// `NotificationTasks.chatTag`, which delegates here).
+/// Canonical per-chat notification tag.
 String chatNotificationTag(String chatToken) => 'talk_$chatToken';
 
 /// Deterministic non-negative 31-bit notification id per chat, so every
 /// message of the same conversation updates ONE stacked notification instead
 /// of adding a new card per push.
+final _fileSafeToken = RegExp(r'^[A-Za-z0-9_-]+$');
+
+/// File-/doc-safe name for a chat token: normally URL-safe and used verbatim,
+/// anything else is base64url-encoded so exotic ids can't escape a directory.
+String fileSafeToken(String chatToken) => _fileSafeToken.hasMatch(chatToken)
+    ? chatToken
+    : base64Url.encode(utf8.encode(chatToken)).replaceAll('=', '');
+
 int stableChatNotificationId(String chatToken) {
   var hash = 0;
   for (final unit in chatToken.codeUnits) {
@@ -128,12 +135,8 @@ class ChatThreadStore {
 
   ChatThreadStore({Localstore? db}) : _db = db ?? Localstore.instance;
 
-  /// Localstore doc ids become file names — tokens are normally URL-safe and
-  /// used verbatim, anything else is base64url-encoded.
-  static String docIdForToken(String chatToken) =>
-      RegExp(r'^[A-Za-z0-9_-]+$').hasMatch(chatToken)
-      ? chatToken
-      : base64Url.encode(utf8.encode(chatToken)).replaceAll('=', '');
+  /// Localstore doc ids become file names.
+  static String docIdForToken(String chatToken) => fileSafeToken(chatToken);
 
   Future<List<ThreadMessage>> messages(String chatToken) async {
     final data = await _doc(chatToken).get();
@@ -145,16 +148,6 @@ class ChatThreadStore {
         .toList();
   }
 
-  /// Appends [message] and returns the updated (capped) history.
-  Future<List<ThreadMessage>> append(
-    String chatToken,
-    ThreadMessage message,
-  ) async {
-    final updated = appendThreadMessage(await messages(chatToken), message);
-    await _write(chatToken, updated);
-    return updated;
-  }
-
   /// Appends [message], but restarts the thread first when the chat's previous
   /// notification is no longer active ([isActive] == false — dismissed/read).
   /// See [threadAfterIncoming] for the tri-state semantics.