feat(layout): context-switcher for multi-role users

Adds components/shared/ContextSwitcher.vue — a Vuetify menu-button
that renders only when useAuthStore.showContextSwitcher is true (i.e.
the user has both portal and organizer contexts available). Click
calls useAuthStore.setLastContext + resolveLandingRoute and pushes
the new route.

Wired into both layouts:
- PortalLayout.vue: navbar right section, before UserAvatarMenu
- DefaultLayoutWithVerticalNav.vue (organizer navbar host): before
  NavbarThemeSwitcher (OrganizerLayout.vue itself is a 10-line
  wrapper around DefaultLayoutWithVerticalNav, so the component
  wires into the actual navbar host).

Boundaries matrix update: components-shared now allows `stores` so
canonical shared chrome (ContextSwitcher, future global indicators)
can read useAuthStore directly without re-homing to
components/layout/. stores-portal stays disallowed for components-
shared by design — portal-specific state has no place in shared
chrome.

Adds 3 vitest specs covering: visibility gated by
showContextSwitcher, click invokes setLastContext + router.push.

Test count 189 → 192. Frontend lint + typecheck clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/app/src/layouts/PortalLayout.vue

@@ -8,6 +8,7 @@
 
 import { useRoute, useRouter } from 'vue-router'
 import type AppLoadingIndicator from '@/components/AppLoadingIndicator.vue'
+import ContextSwitcher from '@/components/shared/ContextSwitcher.vue'
 import UserAvatarMenu from '@/components/portal/UserAvatarMenu.vue'
 import { useAuthStore } from '@/stores/useAuthStore'
 import { usePortalStore } from '@/stores/portal/usePortalStore'
@@ -138,8 +139,9 @@ async function logout() {
 
         <VSpacer />
 
-        <!-- Right section: Avatar menu (desktop) -->
+        <!-- Right section: context switcher + avatar menu (desktop) -->
         <div class="d-none d-md-flex align-center">
+          <ContextSwitcher class="me-2" />
           <UserAvatarMenu />
         </div>