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/Services/NasSyncService.php

@@ -1333,7 +1333,7 @@ class NasSyncService
     /**
      * Rename a path on the NAS share (works for both files and directories).
      */
-    private function renameNasPath(string $oldNasRelPath, string $newNasRelPath): void
+    public function renameNasPath(string $oldNasRelPath, string $newNasRelPath): void
     {
         $cfg    = $this->cfg();
         $target = escapeshellarg($this->smbTarget($cfg));