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>

CODE-AUDIT.md

@@ -473,7 +473,7 @@ token system + shared primitives (UI/SIMP — high value, larger effort) · i18n
   "Reinstall AeroFetch, or drop…" vs "Download it into resources/bin/…" vs "Drop it into resources/bin/."
 - [x] **L47 — `probeMeta` 30s timeout returns null silently** — no surfaced message, unlike every
   other yt-dlp timeout ("Timed out …").
-- [ ] **L48 — `cleanError` strips a leading `error:`** for live failures, while raw `ERROR:` text
+- [x] **L48 — `cleanError` strips a leading `error:`** for live failures, while raw `ERROR:` text
   shows elsewhere (errorlog seed / terminal) — inconsistent error normalization.
 - [x] **L49 — Newlines in user-facing error strings.** download.ts missing-binary messages embed
   `\n`, which renders awkwardly in inline/Caption error spans.