fix(web): repair app-shell landmarks and navigation state

[jeanduplessis]

Problem

Authenticated and admin shells have invalid or incomplete navigation semantics: nested main landmarks, no skip link, inaccessible sidebar rail, undersized trigger, missing current-page state, and noncanonical topbar geometry.

Remediation action

Align shell landmarks and navigation semantics with accessible app-shell guidance.

Scope

• apps/web/src/app/(app)/layout.tsx:20
• apps/web/src/app/(app)/layout.tsx:22
• apps/web/src/components/ui/sidebar.tsx:271
• apps/web/src/components/ui/sidebar.tsx:284
• apps/web/src/app/admin/components/AdminPage.tsx:22
• apps/web/src/app/admin/components/AppSidebar.tsx:310

Acceptance criteria

• Each shell exposes exactly one main landmark with stable main-content target.
• A focus-revealed skip link is first in keyboard order.
• Sidebar triggers/rails are named, focusable where interactive, visibly focused, and touch safe.
• Current navigation item exposes aria-current="page" and canonical selected surface.
• Product topbars use canonical 56px height and border/surface treatment.
• Keyboard traversal through shell navigation is covered by automated tests.