fix: normalize orphaned random-custom completion sound on rehydrate

Adds a guard in the persist merge function so that if completionSound is
"random-custom" but customSounds is empty, it falls back to "none". Prevents
the Select trigger rendering a ghost value after a partial rehydrate or
app-version transition that leaves the library empty.

Generated-By: PostHog Code
Task-Id: 2e7e511c-6b31-4480-8ac1-37863ddd7c3b

Author: pauldambra

packages/ui/src/features/settings/settingsStore.ts

@@ -469,6 +469,12 @@ export const useSettingsStore = create<SettingsStore>()(
         if ((merged.autoConvertLongText as string) === "500") {
           (merged as Record<string, unknown>).autoConvertLongText = "1000";
         }
+        if (
+          merged.completionSound === "random-custom" &&
+          (!merged.customSounds || merged.customSounds.length === 0)
+        ) {
+          (merged as Record<string, unknown>).completionSound = "none";
+        }
         return merged;
       },
     },