user [nfc]: Move selfUserId to PerAccountStoreBase

This `selfUserId` getter has 16 other references across 11 files.
Thankfully our architecture, and in particular this way of combining
the substores:

  class PerAccountStore extends PerAccountStoreBase with ChangeNotifier,
    EmojiStore, UserStore, ChannelStore, MessageStore {

has meant that those all refer to it on the main PerAccountStore type,
rather than on an individual substore like UserStore.  So none of
those need to be touched when we rearrange like this where the data
is stored.

Author: gnprice

lib/model/store.dart

@@ -339,12 +339,20 @@ class CorePerAccountStore {
     required GlobalStore globalStore,
     required this.connection,
     required this.accountId,
+    required this.selfUserId,
   }) : _globalStore = globalStore,
        assert(connection.realmUrl == globalStore.getAccount(accountId)!.realmUrl);
 
   final GlobalStore _globalStore;
   final ApiConnection connection; // TODO(#135): update zulipFeatureLevel with events
   final int accountId;
+
+  // This isn't strictly needed as a field; it could be a getter
+  // that uses `_globalStore.getAccount(accountId)`.
+  // But we denormalize it here to save a hash-table lookup every time
+  // the self-user ID is needed, which can be often.
+  // It never changes on the account; see [GlobalStore.updateAccount].
+  final int selfUserId;
 }
 
 /// A base class for [PerAccountStore] and its substores,
@@ -384,6 +392,14 @@ abstract class PerAccountStoreBase {
   /// which is possible only if [PerAccountStore.dispose] has been called
   /// on this store.
   Account get account => _globalStore.getAccount(accountId)!;
+
+  /// The user ID of the "self-user",
+  /// i.e. the account the person using this app is logged into.
+  ///
+  /// This always equals the [Account.userId] on [account].
+  ///
+  /// For the corresponding [User] object, see [UserStore.selfUser].
+  int get selfUserId => _core.selfUserId;
 }
 
 const _tryResolveUrl = tryResolveUrl;
@@ -441,6 +457,7 @@ class PerAccountStore extends PerAccountStoreBase with ChangeNotifier, EmojiStor
       globalStore: globalStore,
       connection: connection,
       accountId: accountId,
+      selfUserId: account.userId,
     );
     final channels = ChannelStoreImpl(initialSnapshot: initialSnapshot);
     return PerAccountStore._(
@@ -464,9 +481,7 @@ class PerAccountStore extends PerAccountStoreBase with ChangeNotifier, EmojiStor
         typingStartedWaitPeriod: Duration(
           milliseconds: initialSnapshot.serverTypingStartedWaitPeriodMilliseconds),
       ),
-      users: UserStoreImpl(
-        selfUserId: account.userId,
-        initialSnapshot: initialSnapshot),
+      users: UserStoreImpl(core: core, initialSnapshot: initialSnapshot),
       typingStatus: TypingStatus(
         selfUserId: account.userId,
         typingStartedExpiryPeriod: Duration(milliseconds: initialSnapshot.serverTypingStartedExpiryPeriodMilliseconds),
@@ -602,9 +617,6 @@ class PerAccountStore extends PerAccountStoreBase with ChangeNotifier, EmojiStor
   ////////////////////////////////
   // Users and data about them.
 
-  @override
-  int get selfUserId => _users.selfUserId;
-
   @override
   User? getUser(int userId) => _users.getUser(userId);
 

lib/model/user.dart

@@ -2,17 +2,10 @@ import '../api/model/events.dart';
 import '../api/model/initial_snapshot.dart';
 import '../api/model/model.dart';
 import 'localizations.dart';
+import 'store.dart';
 
 /// The portion of [PerAccountStore] describing the users in the realm.
-mixin UserStore {
-  /// The user ID of the "self-user",
-  /// i.e. the account the person using this app is logged into.
-  ///
-  /// This always equals the [Account.userId] on [PerAccountStore.account].
-  ///
-  /// For the corresponding [User] object, see [selfUser].
-  int get selfUserId;
-
+mixin UserStore on PerAccountStoreBase {
   /// The user with the given ID, if that user is known.
   ///
   /// There may be other users that are perfectly real but are
@@ -80,19 +73,16 @@ mixin UserStore {
 /// Generally the only code that should need this class is [PerAccountStore]
 /// itself.  Other code accesses this functionality through [PerAccountStore],
 /// or through the mixin [UserStore] which describes its interface.
-class UserStoreImpl with UserStore {
+class UserStoreImpl extends PerAccountStoreBase with UserStore {
   UserStoreImpl({
-    required this.selfUserId,
+    required super.core,
     required InitialSnapshot initialSnapshot,
   }) : _users = Map.fromEntries(
          initialSnapshot.realmUsers
          .followedBy(initialSnapshot.realmNonActiveUsers)
          .followedBy(initialSnapshot.crossRealmBots)
          .map((user) => MapEntry(user.userId, user)));
 
-  @override
-  final int selfUserId;
-
   final Map<int, User> _users;
 
   @override