feat: Spotify-style shuffle — pre-shuffle queue, each song plays once

Shuffle now shuffles the queue array once (current song first, rest
randomised behind it) instead of picking a random index on every skip.
Toggling shuffle off restores the original queue order. Removed the
'once' repeat state added in the previous commit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/state.js

@@ -5,6 +5,7 @@ export const S = {
   // Playback
   queue: [],
   qIdx: -1,
+  _origQueue: null,
   shuffle: false,
   repeat: false,
   radioMode: false,