feat: PM panel under ERP shell + project-supervisor role

- /panel/pm: the PM app (served on the same origin under /pm) is embedded as
  a full-height iframe; supervisors are funneled there from every route,
  ERP MobileTabBar hidden on that route
- Panels menu: «مدیریت پروژه (PM)» is an internal route (admin/pm/supervisor
  only — procurement managers do not see it); PM link is no longer external
- New system role 'supervisor' (user_role enum): label in users router,
  allowed in UserRoleUpdate schema, deny-by-default on all ERP routers;
  admin panel can grant/revoke it alongside procurement manager
- Removed the obsolete VITE_PM_URL build plumbing (same-origin now)

backend/app/routers/users.py

@@ -41,6 +41,8 @@ USER_ROLE_LABELS = {
     "pm": "مدیر پروژه",
     "procurement_manager": "مدیر تدارکات",
     "admin": "ادمین",
+    # کاربر فقطِ پنل مدیریت پروژه (PM) — به انبار/تدارکات دسترسی ندارد
+    "supervisor": "سرپرست پروژه",
 }