fix(sidebar): keep transient worktree selections stable

Synthetic sidebar worktree rows are transient selections and are not present in the saved repositories list. Repository-store updates were treating those selections as missing and falling back to the previously selected saved repository, which could jump the UI back to an unrelated main repository after selecting a worktree from another repo family.

Preserve transient synthetic worktree selections across repository list updates instead of replacing them during selection reconciliation.

Validation:
- yarn eslint app/src/lib/stores/app-store.ts
- yarn compile:prod

Author: ignatremizov

app/src/lib/stores/app-store.ts

@@ -3033,7 +3033,20 @@ export class AppStore extends TypedBaseStore<IAppState> {
             r.id === selectedRepository.id
         ) || null
 
-      newSelectedRepository = r
+      if (r !== null) {
+        newSelectedRepository = r
+      } else if (
+        selectedRepository instanceof Repository &&
+        selectedRepository.id < 0
+      ) {
+        // Synthetic sidebar-only worktree rows are transient selections and
+        // won't exist in the saved repositories list. Preserve the current
+        // selection across repository store updates instead of falling back to
+        // the previously selected saved repository.
+        newSelectedRepository = selectedRepository
+      } else {
+        newSelectedRepository = null
+      }
     }
 
     if (newSelectedRepository === null && this.repositories.length > 0) {