Bug 43091: (RM follow-up) Give the OPAC self-renewal island a Pinia store

hydrate() only installs Pinia for an island when its config.stores is
non-empty. patron-self-renewal kept config: {} after islands-opac.ts
was split out, so it never got one.

http-client.js's post() calls submitting(), which needs Pinia's
useMainStore(). With none active, it throws before fetch() runs -
get() skips submitting() by default, so only the actual renewal
submission failed.

Test plan:
prove t/cypress/integration/Islands/SelfRenewal.ts

Signed-off-by: Pedro Amorim <pedro.amorim@openfifth.co.uk>

koha-tmpl/intranet-tmpl/prog/js/vue/modules/islands-opac.ts

@@ -1,5 +1,11 @@
 export * from "./islands";
-import { hydrate, registerIsland, WebComponentDynamicImport } from "./islands";
+import {
+    hydrate as hydrateIslands,
+    registerIsland,
+    IslandStoreDefinitions,
+    WebComponentDynamicImport,
+} from "./islands";
+import { useMainStore } from "../stores/main";
 
 /**
  * The islands available in the OPAC.
@@ -16,14 +22,35 @@ const islands: Map<string, WebComponentDynamicImport> = new Map([
                 );
                 return module.default;
             },
-            config: {},
+            config: {
+                stores: ["mainStore"],
+            },
         },
     ],
 ]);
 
+/**
+ * The Pinia stores islands in the OPAC may request via config.stores.
+ * @type {IslandStoreDefinitions}
+ */
+const storeDefinitions: IslandStoreDefinitions = {
+    mainStore: useMainStore,
+};
+
 // Core islands take the same registration path as plugin islands.
 islands.forEach((entry, name) => registerIsland(name, entry));
 
+/**
+ * Hydrates islands with the OPAC store set.
+ *
+ * This local export takes precedence over the star re-export from ./islands,
+ * so imports of hydrate from the built islands.esm.js get the stores baked in.
+ * @returns {void}
+ */
+export function hydrate(): void {
+    hydrateIslands(storeDefinitions);
+}
+
 if (parseInt(document?.currentScript?.getAttribute("init") ?? "0", 10)) {
     hydrate();
 }