gh-12897: prevent live folder dismissed tabs from reappearing

[genu]

Dismissed live folder tabs (e.g. GitHub PRs) would reappear on startup. Two issues caused this:

1. Empty fetch wipes dismissals: onLiveFolderFetch cleaned up dismissed items by removing any not in the current fetch results. If a fetch returned an empty list (auth expired, HTML structure changed), all dismissals were cleared — causing every PR to reappear on the next successful fetch.

2. saveState early return: If any single folder's DOM element wasn't found, the entire saveState method returned early, preventing dismissed items for all folders from being persisted.

Fix: guard the dismissed-items cleanup with itemIds.size > 0, and change return to continue in saveState.

[mr-cheffy]

Does this even run if there has been an error while fetching? Also, is there an existing issue for this? If not, please create one if you dont mind

[genu]

To answer your question — no, this cleanup code does not run when fetchItems() returns an error. Error results (strings like "zen-live-folder-github-no-auth") cause onLiveFolderFetch to return early at the top of the method, before reaching the cleanup loop.

The guard is needed for a different case: when the fetch succeeds (HTTP 200) but the HTML does not contain the expected structure — e.g. GitHub auth expired with a redirect instead of a 404, or a rate-limiting page. In that case fetchItems() returns an empty array (not an error string), and without the guard, the cleanup loop would wipe all dismissals.

Created #12894 for tracking.

[mr-cheffy]

Your explanation doesn't make much sense and it just looks like something AI would spit out to justify the change (GitHub auth expired with a redirect instead of a 404. In that case, the response code wouldnt be 200 anyways), but it doesn't seem to make much harm so its fine.