Search and paginate saved documents in the dashboard #49

Part of #5. Build on the literal document search API merged in #48.

Add a labelled search form to the Documents tab, send the trimmed query through the existing API client, and paginate matching documents. Reset the page and selected document when a search is applied or cleared, ignore stale responses, and show loading, empty, error and retry states.

Verify query encoding, pagination, clearing, errors and stale responses with offline UI tests. Capture the changed dashboard with fixture data. No live Slack access is required.