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/videos/partials/video-details.blade.php

@@ -62,6 +62,11 @@
                 <button class="action-btn" onclick="openEditVideoModal('{{ $video->getRouteKey() }}')">
                     <i class="bi bi-pencil"></i> <span>Edit</span>
                 </button>
+                @if($video->type === 'music')
+                    <button class="action-btn" onclick="openMergeModal('{{ $video->getRouteKey() }}')">
+                        <i class="bi bi-signpost-split"></i> <span>Merge</span>
+                    </button>
+                @endif
             @endif
         @else
             <a href="{{ route('login') }}" class="action-btn">