fix: preserve BrowseList selection during polling

[KalleBylin]

Why

Follow-up to #17. During review there, a remaining BrowseList issue was called out as a good follow-up: background polling did not start after the first successful load, and when polling ran later it dropped the current selection.

What changed

• arm background polling after the first successful load instead of gating it only on a ref set during the same render cycle
• track BrowseList selection by wanted item id rather than row index
• preserve selection across background refreshes when the same item still exists, and clear it only if it disappears
• add focused regression coverage around poll startup and selection preservation across refreshes

Local verification

Checked locally with the web test suite and frontend checks.

web run check still reports the existing BrowseList.module.css descending-specificity warning, unchanged by this PR.