feat(studio): module-store manager (list / add / remove)

A store icon in the Store app bar opens a dialog that lists the
configured module stores + the bundled seed (with per-source module
counts), lets the operator add a store by index URL (the hub fetches +
merges it live so its modules appear immediately), and remove a store.
Backed by the new ListStores/AddStore/RemoveStore RPCs + SDK methods.
This is how a domain app's published modules (e.g. reclaim's) become
visible in the Store without touching the CLI.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

lib/data/hub.dart

@@ -212,6 +212,21 @@ class HubService {
 
   Future<bool> healthy() => _client.healthy();
 
+  /// Configured module stores (+ the bundled seed) for the store manager.
+  Future<List<StoreSource>> listStores() => _client.listStores();
+
+  /// Register a new module store; the hub fetches + merges it live.
+  Future<AddStoreResponse> addStore({
+    required String name,
+    required String url,
+    String bearerEnv = '',
+  }) =>
+      _client.addStore(name: name, url: url, bearerEnv: bearerEnv);
+
+  /// Drop a configured store by name (the bundled seed cannot be removed).
+  Future<RemoveStoreResponse> removeStore(String name) =>
+      _client.removeStore(name);
+
   /// List of installed WASM modules, grouped by `module_name`.
   /// Built-in and federated capabilities are excluded — they
   /// don't correspond to a bundle on disk and would confuse the