refactor(layouts): shared RoleSwitcherPill across dashboards (§D.6)

Extract ROLE_SWITCH_OPTIONS + switchableRoles computation from the two
duplicated copies (role-nav.tsx + admin-sidebar.tsx) into a single
src/components/layouts/role-switcher.tsx module.

Adds a RoleSwitcherPill component placed top-right of every dashboard:
  - Hidden for single-role users
  - Hidden during impersonation
  - Same visual + click target across /jury, /mentor, /applicant,
    /observer, /award-master AND /admin (admin layout gains a small
    top-bar to host the pill)

Removes the duplicate role-switcher items from the admin sidebar's
bottom user-menu — one source of truth instead of three.

Plan: docs/superpowers/plans/2026-04-28-pr6-multi-role-and-workspace-previews.md

src/app/(admin)/layout.tsx

@@ -2,6 +2,7 @@ import { prisma } from '@/lib/prisma'
 import { requireRole } from '@/lib/auth-redirect'
 import { AdminSidebar } from '@/components/layouts/admin-sidebar'
 import { AdminEditionWrapper } from '@/components/layouts/admin-edition-wrapper'
+import { RoleSwitcherPill } from '@/components/layouts/role-switcher'
 
 export default async function AdminLayout({
   children,
@@ -34,6 +35,12 @@ export default async function AdminLayout({
         <main className="lg:pl-64">
           {/* Spacer for mobile header */}
           <div className="h-16 lg:hidden" />
+          {/* Top-bar — hosts the RoleSwitcherPill so multi-role admins
+              can switch dashboards from the same screen position used on
+              every other layout. Pill auto-hides for single-role users. */}
+          <div className="sticky top-0 z-30 flex h-12 items-center justify-end gap-2 border-b bg-card/80 backdrop-blur px-4">
+            <RoleSwitcherPill currentBasePath="/admin" />
+          </div>
           <div className="container-app py-6 lg:py-8">{children}</div>
         </main>
       </div>