feat(discord-rpc): wire initDiscordRpc into app boot

Calls discordRpcService.initDiscordRpc() once from +page.svelte onMount so
the backend state matches the persisted toggle and the playerStore subscriber
is registered when the user has opted in.

Refs #336.

cc

src/routes/+page.svelte

@@ -517,6 +517,7 @@
     resetLaunchFlow,
   } from '$lib/services/updatesService';
   import type { AutoUpdateProgress } from '$lib/services/updatesService';
+  import { initDiscordRpc } from '$lib/services/discordRpcService';
   import UpdateProgressModal from '$lib/components/updates/UpdateProgressModal.svelte';
 
   // Offline state
@@ -4603,6 +4604,8 @@
     // Bootstrap app (theme, mouse nav, Last.fm restore)
     const { cleanup: cleanupBootstrap } = bootstrapApp();
 
+    void initDiscordRpc();
+
     // Window-title preference: load from localStorage and subscribe so that
     // toggling the setting updates the OS title bar immediately.
     initWindowTitleStore();