feat: remove admin video jobs link from navigation and update card background style

Co-authored-by: Copilot <copilot@github.com>

frontend/app/main.py

@@ -516,13 +516,6 @@ def admin_models():
     return render_template("admin/models.html")
 
 
-@app.get("/admin/videos")
-@admin_required
-def admin_videos():
-    """Show all video generation jobs across all users."""
-    return render_template("admin/videos.html")
-
-
 # ── Admin API proxies (same-origin for browser JS, avoids mixed-content) ──
 
 @app.get("/api/admin/videos")