Video merge feature, default avatar, video-redirects table, misc UI tweaks

Bundled pre-existing changes on the branch:
- Video merge: controller, service, admin merge modal, and a repair
  console command; new video_redirects table (with source_folder)
  so merged/moved videos keep resolving under their old keys.
- Default avatar SVG for users without a profile picture.
- Assorted view tweaks across profile, video actions/comments/insights,
  admin layout, video-details, and the app layout.
- Small touch-ups in SuperAdminController, User model, NasSyncService.

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

resources/views/components/video-actions.blade.php

@@ -128,6 +128,12 @@
                 <i class="bi bi-pencil"></i>
                 <span>Edit</span>
             </button>
+            @if($video->type === 'music')
+                <button class="action-btn desktop-action" onclick="openMergeModal('{{ $video->getRouteKey() }}')">
+                    <i class="bi bi-signpost-split"></i>
+                    <span>Merge</span>
+                </button>
+            @endif
             {{-- Lyrics generate/regenerate + edit now live inside the player's gear menu
                  so they're always reachable on both mobile and desktop. --}}
             <button class="action-btn desktop-action" onclick="showDeleteModal('{{ $video->getRouteKey() }}', {{ json_encode($video->title) }})"
@@ -250,6 +256,11 @@
                     <button type="button" class="dropdown-item" onclick="openEditVideoModal('{{ $video->getRouteKey() }}')">
                         <i class="bi bi-pencil"></i> Edit
                     </button>
+                    @if($video->type === 'music')
+                        <button type="button" class="dropdown-item" onclick="openMergeModal('{{ $video->getRouteKey() }}')">
+                            <i class="bi bi-signpost-split"></i> Merge
+                        </button>
+                    @endif
                 @endif
             @else
                 <button class="dropdown-item" onclick="window.location.href='{{ route('login') }}'">