fix(main): R9 — guard the yt-dlp update throttle against a backward clock

shouldAutoCheckYtdlp compared (now - lastCheck) >= interval; a backward system-
clock correction leaves lastCheck in the future so the difference is negative and
the daily check never comes due again. Treat a future lastCheck as skewed and run
the check. The renderer scheduled-item promoter is inherently fire-once (promotion
flips the item out of 'saved'), so it can't double-fire on a backward jump — a
forward jump firing early is documented and accepted as minor.

Unit-tested. typecheck + 263 tests + eslint + prettier green.

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

src/renderer/src/store/downloads.ts

@@ -272,6 +272,10 @@ export const useDownloads = create<DownloadState>((set, get) => {
     },
 
     promoteDueScheduled: () => {
+      // Wall-clock comparison: each scheduled item fires once (the promotion flips it
+      // out of 'saved'), so a backward clock jump only delays it and can't double-fire;
+      // a forward jump can fire it early. Accepted as minor per R9 — a monotonic
+      // scheduler would be overkill for a park-until-time feature.
       const now = Date.now()
       set((s) => ({
         items: s.items.map((i) =>