fix(ui): show real error message (not [object Object]) on OAuth link failure

src/lib/stores/settings.svelte.ts

@@ -5,6 +5,7 @@
 
 import {
   api,
+  errorMessage,
   events,
   type AppSettings,
   type SyncTargetInfo,
@@ -104,7 +105,7 @@ class SettingsStore {
       window.open(authUrl, "_blank");
       this.linkMessage = "Finish sign-in in your browser…";
     } catch (e) {
-      this.linkMessage = `Link failed: ${e}`;
+      this.linkMessage = `Link failed: ${errorMessage(e)}`;
     }
   }