fix: track a playback currentIndex so prev and repeat-all work

next() did `queue.slice(idx + 1)`, so the current track was always
queue[0]. prev()'s `idx > 0` guard could therefore never pass after an
auto-advance — Previous did nothing, ever — and repeat: 'all' jumped to
queue[0], which is the track that just finished, looping the last track of
an album instead of restarting it.

Replaced with a currentIndex cursor; the queue is no longer trimmed behind
the playhead. The old slice did serve a purpose — bounding Vibe-prefetch
growth — so that is preserved as a MAX_HISTORY = 50 cap that drops the
oldest entries and re-bases the index, rather than dropped outright.
setQueue/playTrack/setCurrentTrack recompute the cursor, next()/prev() fall
back to findIndex if it drifts, and shuffle now picks by index so the
cursor stays valid.

Consumer audit: NowPlayingPanel and Vibe.tsx already derived position via
findIndex and needed no change. TrackRow.handlePlay did
`setQueue(queue.slice(index))`, which re-broke prev at the point of click
even with the store fixed; it now passes the intact queue.

This commit also includes a pre-existing uncommitted fix from the working
tree (not authored by Claude): the end-of-queue auto-resume loop, which
stops playback at the end of the queue instead of restarting. It is correct
and independent of the cursor bug, and is preserved verbatim here.

REVIEW-2026-07-30.md finding 7.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/components/TrackRow.tsx

@@ -26,7 +26,7 @@ interface TrackRowProps {
   showVibe?: boolean;
 }
 
-export function TrackRow({ track, queue, index, showActions = true, variant = 'default', showVibe = false }: TrackRowProps) {
+export function TrackRow({ track, queue, showActions = true, variant = 'default', showVibe = false }: TrackRowProps) {
   const { setQueue, playTrack, play, pause, currentTrack, isPlaying } = usePlaybackStore();
   const dislikeTrack = useDislikeTrack();
   const router = useRouter();
@@ -35,7 +35,9 @@ export function TrackRow({ track, queue, index, showActions = true, variant = 'd
 
   const handlePlay = () => {
     if (isCurrent) { isPlaying ? pause() : play(); return; }
-    setQueue(queue.slice(index));
+    // Queue the whole list and start at this track, so Previous can walk back
+    // into the tracks before it.
+    setQueue(queue);
     playTrack(track);
   };