The account sub-pages were already protected via validateTokens; add a loader
on the profile index so the hub itself bounces to /login when unauthenticated.
- new profile.tsx layout: sticky desktop sidebar (user card + menu) wrapping all
/profile/* pages; mobile renders page content only (unchanged)
- ProfilePagesHeader hidden at lg (one change covers all account sub-pages +
cart); profile index shows a welcome on desktop (menu lives in the sidebar)
- sellers: 5-col brand grid with name overlay, contained, mobile header hidden
- MyLayout: desktop gate now covers /profile/* and /sellers
- cart checkout (/cart/$cartId): steps on the left, sticky order summary on the
right at lg (1320px container); CartSummary becomes an inline card on desktop
(fixed bottom bar on mobile)
- profile (/profile): contained account layout; menu becomes a 2-col card grid
on desktop with a heading
- MyLayout: desktop gate now includes /cart/* and /profile
