feat: add dedicated admin user edit page with platform management

The admin user edit page now uses a dedicated template and JS file
that properly matches the /admin/admin-users/{id}/edit URL pattern.
The page allows super admins to:
- Toggle super admin status
- Toggle active status
- Manage platform assignments for platform admins
- Delete admin users

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routes/admin_pages.py

@@ -499,7 +499,7 @@ async def admin_user_edit_page(
         raise HTTPException(status_code=403, detail="Super admin access required")
 
     return templates.TemplateResponse(
-        "admin/user-edit.html",
+        "admin/admin-user-edit.html",
         {
             "request": request,
             "user": current_user,