fix(ui): prevent "Play Next" restarting play at top of queue (#5049)

Set playIndex when rebuilding the queue in reducePlayNext so the music
player library knows which track is currently playing. Without this, the
library's loadNewAudioLists defaults playIndex to 0, causing playback to
restart from the top of the queue on rapid "Play Next" actions.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/reducers/playerReducer.js

@@ -127,10 +127,12 @@ const reducePlayNext = (state, { data }) => {
   const newQueue = []
   const current = state.current || {}
   let foundPos = false
+  let currentIndex = 0
   state.queue.forEach((item) => {
     newQueue.push(item)
     if (item.uuid === current.uuid) {
       foundPos = true
+      currentIndex = newQueue.length - 1
       Object.keys(data).forEach((id) => {
         newQueue.push(mapToAudioLists(data[id]))
       })
@@ -145,6 +147,7 @@ const reducePlayNext = (state, { data }) => {
   return {
     ...state,
     queue: newQueue,
+    playIndex: foundPos ? currentIndex : undefined,
     clear: true,
   }
 }