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/layouts/app.blade.php

@@ -1230,6 +1230,7 @@
     @include('layouts.partials.upload-modal')
     @include('layouts.partials.edit-video-modal')
     @include('layouts.partials.sports-match-modal')
+    @include('layouts.partials.merge-video-modal')
     @endauth
 
     <!-- Add to Playlist Modal - Available for all users (shows login prompt if not authenticated) -->