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/user/profile.blade.php

@@ -330,7 +330,7 @@ if ($oldLinks) {
                 @if($user->avatar)
                     <img src="{{ route('media.avatar', $user->avatar) }}" alt="{{ $user->name }}" class="profile-avatar" id="pageAvatar">
                 @else
-                    <img src="https://i.pravatar.cc/150?u={{ $user->id }}" alt="{{ $user->name }}" class="profile-avatar" id="pageAvatar">
+                    <img src="{{ $user->avatar_url }}" alt="{{ $user->name }}" class="profile-avatar" id="pageAvatar">
                 @endif
             </div>
 
@@ -527,7 +527,7 @@ if ($oldLinks) {
                                 @if($user->avatar)
                                     <img src="{{ route('media.avatar', $user->avatar) }}" class="avatar-preview" id="avatarPreview" alt="Avatar">
                                 @else
-                                    <img src="https://i.pravatar.cc/150?u={{ $user->id }}" class="avatar-preview" id="avatarPreview" alt="Avatar">
+                                    <img src="{{ $user->avatar_url }}" class="avatar-preview" id="avatarPreview" alt="Avatar">
                                 @endif
                                 <button type="button" class="avatar-edit-btn" id="avatarEditBtn" title="Change photo">
                                     <i class="bi bi-camera-fill"></i>