SPA playlist transitions — no page refresh on track change

- Add GET /videos/{video}/player-data JSON endpoint returning stream URL,
  cover, slides, title, duration (used by client-side SPA transitions)
- Replace music playlist JS with full SPA system: plTransitionTo() swaps
  audio.src in-place (preserving browser autoplay permission), updates
  cover art, resets progress bar, then background-fetches the new page
  to swap #vdbWrap (description) and #ytcSection (comments) via DOMParser
- plSwapContent() re-runs the YTC comments IIFE after swapping innerHTML
  so comments load correctly for the new video
- Prev/next/shuffle/loop/autoplay controls now computed dynamically from
  PL_VIDEOS array — buttons stay correct after each SPA transition
- Sidebar shows ALL playlist tracks (removed @if filter); current track
  highlighted in red; clicking any card triggers SPA transition
- Browser back/forward handled via popstate + history.pushState

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

app/Http/Controllers/VideoController.php

@@ -465,6 +465,33 @@ class VideoController extends Controller
             ->withCookie(cookie('_did', $did, 60 * 24 * 365 * 5));
     }
 
+    public function playerData(Video $video, Request $request)
+    {
+        if (! $video->canView(Auth::user())) {
+            abort(403);
+        }
+
+        $coverUrl = $video->thumbnail
+            ? route('media.thumbnail', $video->thumbnail)
+            : asset('storage/images/logo.png');
+
+        $slides = $video->slides->count() > 1
+            ? $video->slides->map(fn ($s) => route('media.thumbnail', $s->filename))->values()->all()
+            : [];
+
+        return response()->json([
+            'id'         => $video->id,
+            'key'        => $video->getRouteKey(),
+            'type'       => $video->type,
+            'stream_url' => route('videos.stream', $video),
+            'cover_url'  => $coverUrl,
+            'slides'     => $slides,
+            'title'      => $video->title,
+            'author'     => $video->user->name ?? '',
+            'duration'   => $video->duration,
+        ]);
+    }
+
     public function matchData(Video $video)
     {
         if (! $video->canView(Auth::user())) {