Fix Explore filter accessibility and empty-result recovery #143

Part of #85.

Explore currently renders two empty messages when a selected category has no search matches. Its search field has no visible label, category selection is only visual, and there is no single action to restore the tool list.

This slice will add a labelled responsive search field, expose category selection and the result count to assistive technology, and render one empty state with Clear filters that resets search and category and returns focus to search. Keep the existing tools and matching rules.

Acceptance criteria

• One empty message for both all-category and selected-category searches.
• Clear filters restores all tools, selects All and focuses the search input.
• Component tests cover combined filtering and recovery.
• Desktop and narrow-screen browser proof is embedded in the PR.

The parent demo-readiness epic remains open.