feat: add keyboard selection for slash completions

[miloquinn]

Summary

• Track the selected slash-command completion in shared UI state
• Use Up/Down to move through visible slash completions with wraparound
• Use Enter to fill the selected command into the input
• Use Escape to dismiss the completion list without triggering the clear-input prompt
• Highlight the selected completion with the same marker pattern as file suggestions

Why

The slash completion menu was display-only unless there was exactly one match. When multiple commands matched, users could not navigate the menu or select a highlighted command with Enter, even though the UI already showed a completion list.

This implements the keyboard-selection behavior requested in #575 while preserving existing file-autocomplete priority.

Testing

• npm run test:types
• npx ava source/hooks/useUIState.spec.ts source/components/user-input.spec.tsx --timeout=60s
• npm run test:format -- source/hooks/useUIState.ts source/hooks/useUIState.spec.ts source/components/user-input.tsx source/components/user-input.spec.tsx
• git diff --check

Note: test:format reports existing Biome config schema/deprecation infos, but no formatting errors after this change.