play-queue: peekNext() — expose the following track for queue-aware UIs (#719)

A results screen that offers "Up next: <song> — starting in 10s" needs to
know WHAT follows without reaching into queue internals. peekNext() returns
{filename, index, total} for the next track (null when nothing follows),
pure — peeking never plays or mutates.

First consumer: the note_detect results card's queue-advance strip (the
"Playlist Play All has no way to progress" tester issue).


Claude-Session: https://claude.ai/code/session_01Nm7tHs1Yvjjtnnu4nzJgdN

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

static/app.js

@@ -6778,6 +6778,14 @@ window.feedBack.playQueue = (function () {
         start: start, advance: advance, hasNext: hasNext, active: active, clear: clear,
         source: function () { return source; },
         remaining: function () { return active() ? list.length - idx - 1 : 0; },
+        // What's coming, for consumers that RENDER the queue (a results
+        // screen's "Up next: … starting in 10s" strip) without reaching into
+        // queue internals. Null when nothing follows.
+        peekNext: function () {
+            return hasNext()
+                ? { filename: list[idx + 1], index: idx + 1, total: list.length }
+                : null;
+        },
     };
 })();