user [nfc]: Add a selfUser getter

Author: gnprice

lib/model/user.dart

@@ -9,6 +9,8 @@ mixin UserStore {
   /// 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;
 
   /// All known users in the realm, by [User.userId].
@@ -31,6 +33,12 @@ mixin UserStore {
   /// Consider using [userDisplayName].
   Map<int, User> get users;
 
+  /// The [User] object for the "self-user",
+  /// i.e. the account the person using this app is logged into.
+  ///
+  /// When only the user ID is needed, see [selfUserId].
+  User get selfUser => users[selfUserId]!;
+
   /// The name to show the given user as in the UI, even for unknown users.
   ///
   /// This is the user's [User.fullName] if the user is known,

lib/widgets/compose_box.dart

@@ -1408,9 +1408,8 @@ class _ComposeBoxState extends State<ComposeBox> with PerAccountStoreAwareStateM
       case ChannelNarrow(:final streamId):
       case TopicNarrow(:final streamId):
         final channel = store.streams[streamId];
-        final selfUser = store.users[store.selfUserId]!;
         if (channel == null || !store.hasPostingPermission(inChannel: channel,
-            user: selfUser, byDate: DateTime.now())) {
+            user: store.selfUser, byDate: DateTime.now())) {
           return _ErrorBanner(label:
             ZulipLocalizations.of(context).errorBannerCannotPostInChannelLabel);
         }

lib/widgets/inbox.dart

@@ -385,7 +385,7 @@ class _DmItem extends StatelessWidget {
     final designVariables = DesignVariables.of(context);
 
     final title = switch (narrow.otherRecipientIds) { // TODO dedupe with [RecentDmConversationsItem]
-      [] => store.users[store.selfUserId]!.fullName,
+      [] => store.selfUser.fullName,
       [var otherUserId] => store.userDisplayName(otherUserId),
 
       // TODO(i18n): List formatting, like you can do in JavaScript:

lib/widgets/recent_dm_conversations.dart

@@ -85,9 +85,8 @@ class RecentDmConversationsItem extends StatelessWidget {
     final Widget avatar;
     switch (narrow.otherRecipientIds) { // TODO dedupe with DM items in [InboxPage]
       case []:
-        final selfUser = store.users[store.selfUserId]!;
-        title = selfUser.fullName;
-        avatar = AvatarImage(userId: selfUser.userId, size: _avatarSize);
+        title = store.selfUser.fullName;
+        avatar = AvatarImage(userId: store.selfUserId, size: _avatarSize);
       case [var otherUserId]:
         // TODO(#296) actually don't show this row if the user is muted?
         //   (should we offer a "spam folder" style summary screen of recent