Catalog UI 5/5: keyboard and screen-reader accessibility pass #19

Part of #4. Depends on Catalog UI 1/5 to 3/5; can be built alongside them and verified at the end.

Scope

• Keyboard-accessible filters and selection controls.
• Screen-reader labels, including for filter state, selection state and the reason a dependency was auto-selected or an entry is disabled.
• Visible focus states.

Acceptance criteria

• Every filter and selection control is reachable and operable by keyboard alone.
• Disabled/incompatible entries expose their explanation to assistive technology, not only visually.
• A short manual checklist is recorded in the PR (keyboard-only walkthrough, one screen-reader pass).

Out of scope

Visual redesign.