The wishlist page had no way to empty a tab — only per-item remove. Added a red-tinted
'Clear all' button in the toolbar that empties the ACTIVE tab in one click (after a
confirm), shown only when that tab has items.
- db.clear_wishlist(kind) maps the tab to its rows (movie→'movie', show→'episode',
youtube→'video') and deletes them; returns the count.
- POST /api/video/wishlist/clear {kind: movie|show|youtube}.
- Toolbar button + clearAll() (confirm → clear → reload) + updateClearBtn() visibility.
Tests: per-tab clear leaves the others intact, unknown-kind/empty no-ops, the endpoint,
and the frontend wiring. node --check clean.
