fix(migration): handle already-v5 settings and ensure 'failed' branch writes v5 marker

src/renderer/lib/migrations/v4-to-v5.ts

@@ -10,7 +10,16 @@ export const v4SettingsSchema = z.object({
   snippets: z.array(z.unknown()).default([]),
 });
 
+const v5OnlyFields = ['updateChannel', 'autoCheckUpdates', 'firstRun'];
+
+function isAlreadyV5(data: unknown): boolean {
+  if (!data || typeof data !== 'object') return false;
+  if ((data as Record<string, unknown>)['migration.version'] === 5) return true;
+  return v5OnlyFields.some((f) => f in (data as Record<string, unknown>));
+}
+
 export function migrateV4ToV5(v4: unknown): z.infer<typeof settingsSchema> {
+  if (isAlreadyV5(v4)) return v4 as z.infer<typeof settingsSchema>;
   const parsed = v4SettingsSchema.parse(v4);
   const defaults = settingsSchema.parse({});
   return {