Allow converting a video to a music track without losing views/URL

VideoController@update() now handles a replacement media file: if the
new file is audio the row is forced to type=music and, when the on-disk
type folder differs from the declared type (e.g. a video sitting under
users/{u}/videos/ that was later marked music), the media is relocated
into the canonical users/{u}/music/{slug}/tracks/{lang}-{id}/audio.ext
layout. Old primary file and stale meta.json are deleted from NAS; HLS
and slideshow caches are reset. The same relocation is applied by
replaceFile() so the mobile "Replace Media File" flow behaves the same.

Slide- and audio-track-management gates in update() also switched from
isAudioOnlyFile() (extension-based) to the declared type, so secondary
tracks and cover slides added mid-conversion are no longer silently
dropped. The edit modal now renders existing secondary tracks whenever
type=music regardless of the primary file extension.

Also carries an unrelated app.blade.php tweak: the mobile filter bar's
sticky reset moved from .yt-filter-bar to its wrapper, with padding
kept on the inner bar.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

resources/views/layouts/app.blade.php

@@ -1198,12 +1198,15 @@
             }
             body.has-impersonate-bar .yt-main { top: calc(56px + 40px) !important; }
             .yt-main.video-view-page { padding: 0 !important; }
-            .yt-filter-bar {
+            .yt-filter-bar-wrap {
                 position: relative !important;
                 top: auto !important;
                 z-index: auto !important;
                 margin: -16px -16px 16px !important;
             }
+            .yt-filter-bar {
+                padding: 12px 16px !important;
+            }
         }
     </style>