Unify upload/edit modal track-form + fix slide cropper hang & reorder loss

Track-form unification: the "Add Language Track" dialog in the edit modal now
clones the same #um-track-form-template partial that the create modal uses, so
future tweaks to markup/CSS/behaviour show up in both flows automatically.
Added window.cloneTrackFormTemplate() + window.initTrackFormNode() as the
shared cloner+init. updateTrackCard() writes to both um-tc-*-{id} and
edit-tc-*-{id} ids so one helper serves both modals. Dropped the dead
#edit-extra-track-tpl and the custom editHandleTrackAudio (handleTrackAudio
auto-fills title identically).

Cover slides cropper fixes (both modals):
  - Second batch no longer hangs. _slidesCropLoadNext now calls
    openCropperModal_* BEFORE tcPreload_*; previously preloading first measured
    canvas.clientWidth=0 against a display:none overlay and left _st.ready
    false so Save silently returned.
  - Reorder no longer loses thumbnails. Create flow stored raw Files and
    spawned a FileReader per slide per render; under memory pressure some
    readers failed silently. Now stores {file, url} with URL.createObjectURL
    called once per slide, and renderSlidesStrip assigns img.src = url
    directly. Removal revokes the URL; clear revokes everything it drops.
  - Reset .value on the slides input after processing so re-selecting the
    same filename still fires change.

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

resources/views/components/track-editor-form.blade.php

@@ -306,7 +306,7 @@
                 @if($embedAudioInput)
                 <input type="file" id="edit-{{ $p }}-audio-input" name="{{ $audioInputName }}"
                        accept="audio/*,.mp3,.m4a,.aac,.flac,.wav" style="display:none;"
-                       onchange="document.getElementById('edit-{{ $p }}-fname').textContent=this.files[0]?.name||'Keep existing / choose new…'">
+                       onchange="editHandleTrackAudio(this, '{{ $p }}', '{{ $titleId }}')">
                 @endif
             </div>
         </div>