perf(watch-history): load in background; re-run For You when it resolves

The For You feed awaited watchHistoryService.loadLatest() before rendering, and
loadLatest blocked on the full ~1500-item watch-history fetch on a cold load even
with allowStale:true (the `!allowStale || !hasCachedItems` guard awaited whenever
there was no cache). So the main feed froze behind the entire watch-history
resolution, and nothing re-ran the feed once history finished loading — watched
videos were never suppressed and watch-history tags never applied until a manual
refresh.

- watchHistoryService.loadLatest: allowStale callers now NEVER block. On a cold
  load the refresh is kicked off in the background and the call returns
  immediately with whatever is available (cached items, else the local queue).
- feedCoordinator.subscribeWatchHistoryFeedRefresh: subscribes to the
  "fingerprint" event and, when the For You feed is active, debounced-re-runs it
  so newly-resolved watched videos are suppressed and tags applied. Wired in at
  bootstrap alongside feed registration.

The re-run reads the now-fresh cache (no further refresh scheduled), so there is
no loop. Adds feedCoordinatorWatchHistoryRefresh.test.mjs (debounced single
refresh while active; no refresh when inactive; idempotent subscription).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

js/ui/applicationBootstrap.js

@@ -275,6 +275,8 @@ export default class ApplicationBootstrap {
     app.registerExploreFeed();
     app.registerSubscriptionsFeed();
     app.registerWatchHistoryFeed();
+    // Re-run the For You feed when background watch-history resolution completes.
+    app.subscribeWatchHistoryFeedRefresh();
 
     app.watchHistoryController = new WatchHistoryController({
       watchHistoryService,