store: Replace realmIcon getter with resolvedRealmIcon

We almost always need the full resolved realm icon URL,
so provide a getter that does that.

Author: rajveermalviya

lib/model/store.dart

@@ -426,8 +426,11 @@ abstract class PerAccountStoreBase {
   /// Always equal to `account.realmName`.
   String get realmName => account.realmName!;
 
-  /// Always equal to `account.realmIcon`.
-  Uri get realmIcon => account.realmIcon!;
+  /// The full, resolved URL for the Zulip realm icon.
+  ///
+  /// Returned URL is derived by resolving [account.realmIcon] (relative) URL
+  /// against the base [realmUrl].
+  Uri get resolvedRealmIcon => realmUrl.resolveUri(account.realmIcon!);
 
   /// Resolve [reference] as a URL relative to [realmUrl].
   ///