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/admin/layout.blade.php

@@ -538,7 +538,7 @@
             @if(Auth::user()->avatar)
                 <img src="{{ route('media.avatar', Auth::user()->avatar) }}" class="adm-user-avatar" alt="">
             @else
-                <img src="https://i.pravatar.cc/150?u={{ Auth::user()->id }}" class="adm-user-avatar" alt="">
+                <img src="{{ Auth::user()->avatar_url }}" class="adm-user-avatar" alt="">
             @endif
             <div class="adm-user-info">
                 <div class="adm-user-name">{{ Str::limit(Auth::user()->name, 18) }}</div>
@@ -637,7 +637,7 @@
             @if(Auth::user()->avatar)
                 <img src="{{ route('media.avatar', Auth::user()->avatar) }}" alt="">
             @else
-                <img src="https://i.pravatar.cc/150?u={{ Auth::user()->id }}" alt="">
+                <img src="{{ Auth::user()->avatar_url }}" alt="">
             @endif
             <div class="adm-sidebar-user-info">
                 <div class="adm-sidebar-user-name">{{ Str::limit(Auth::user()->name, 20) }}</div>