[catchouli]

Library and watchlist sorts can now run either way.

• Picking a sort starts in its usual direction (newest first for dates, A to Z for titles). Picking the current sort again flips it.
• The Sort button ends with an arrow (↓ newest first / Z to A, ↑ oldest first / A to Z), and the dropdown names the current direction.
• Undated shows (release date) and never-played ones (recently played) stay last either way.
• "A–Z" is now "Title", so it reads sensibly when reversed.
• Saved per library with the other toolbar choices (posters|added|all|desc). Older settings use the sort's usual direction.
• Web UI library page: an ↑/↓ button next to the sort list (re-picking a <select> option fires no event). Changing the sort resets the direction.
• Version 0.21.0.

Tested: unit tests for the sorting and flipping; screenshots of the app in both directions on the anime library. The web sort was checked with node against real library data; the web button wasn't clicked through in a browser.

Not covered: music album sorts (that toolbar cycles sorts on OK rather than opening a dropdown).