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/pages/store.dart

@@ -234,6 +234,11 @@ class _StorePageState extends State<StorePage> {
               onPressed: _openFilterDialog,
             ),
           ),
+          IconButton(
+            icon: const Icon(Icons.storefront_outlined, size: 18),
+            tooltip: 'Module stores',
+            onPressed: () => ChainStoresDialog.show(context),
+          ),
           IconButton(
             icon: const Icon(Icons.refresh, size: 18),
             tooltip: l.storeReloadTooltip,