An empty /explore with no query says nothing is public yet and how to make the first app, not 'clear the line' #4110

Seen on production: /explore with an empty line on a forge with nothing public says 'Nothing matches. Clear the line to see everything you can.', but the line is already clear.

Done:

• [ ] In src/web/studio/explore.ts, the empty state with no text and no kind: or sort: says 'Nothing public yet', with a btn to /apps/new (or 'Sign in to make one' with /signin?next=/apps/new when signed out). With a query, today's 'Nothing matches' stays.
• [ ] Tests: the empty forge with no query, signed in and signed out, and a query with no hits.

Files: src/web/studio/explore.ts, a test. Risk: #3801 redraws the explore rows; rebase onto it if it lands first. Before pushing: bun x tsc --noEmit and the tests you touched.