Fix L48: strip ERROR: prefix from errorlog seed data to match production

cleanError() strips leading 'ERROR:' from yt-dlp stderr before sending
errors to the UI and error log. The browser-preview seed entry in
store/errorlog.ts had the raw 'ERROR: [youtube]...' prefix, making the
preview inconsistent with what production shows. Removed the prefix from
the seed string so the preview and production rendering match.

typecheck + 242 tests green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/renderer/src/store/errorlog.ts

@@ -13,7 +13,7 @@ const seed: ErrorLogEntry[] = PREVIEW
         title: 'Private video',
         url: 'https://youtube.com/watch?v=err0r',
         kind: 'video',
-        error: 'ERROR: [youtube] err0r: Private video. Sign in if you have access.',
+        error: '[youtube] err0r: Private video. Sign in if you have access.',
         occurredAt: Date.now() - 1000 * 60 * 12
       }
     ]