feat(explorer): stale-dataset triage queue

### Description

Below the dataset table, list datasets with no write in N days, ranked by cost times days since last write. Two dispositions per row: Keep (mute inactivity alerts for this dataset, nothing else) and Terminate (opens the flow from #466).

Keep is an alert mute only. No top-up link, no runway coupling. Without it, the only way to stop the inactivity email is to delete data.

### Expected behavior

N is a constant for v0 (proposed 30 days). Kept datasets drop out of the queue and stop generating email. Queue is empty-state hidden when nothing is stale.

### Additional context

Parent: #460. Depends on #463. Mute state needs to persist per wallet; where (local, notifications API) is open and should be decided with #467.

Merges alone: queue is hidden when the dataset table is absent or nothing is stale; Terminate button is disabled until #466 merges.

Start at the explorer dataset table and trace how stale datasets and inactivity emails are represented. Review the dependency on #463 and the termination flow from #466, then resolve the open mute-persistence question with #467. Done means the queue is ranked correctly, supports Keep and the gated Terminate action, hides when empty or the table is absent, and removes kept datasets from alerts.

Written by the indexing model from the issue text.