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]>

app/Http/Controllers/SuperAdminController.php

@@ -772,7 +772,7 @@ class SuperAdminController extends Controller
                     ->groupBy('users.id', 'users.name', 'users.avatar')
                     ->orderByDesc('video_count')->take($limit)->get();
                 return response()->json(['type' => 'uploaders', 'items' => $items->map(fn($u) => [
-                    'avatar' => $u->avatar ? asset('storage/avatars/'.$u->avatar) : 'https://i.pravatar.cc/40?u='.$u->id,
+                    'avatar' => $u->avatar ? asset('storage/avatars/'.$u->avatar) : asset('images/default-avatar.svg'),
                     'name'   => $u->name,
                     'videos' => $u->video_count,
                     'views'  => $u->total_views,