refactor(auth): merge usePortalAuthStore into useAuthStore with context-aware getters

usePortalAuthStore is deleted — its 114 lines were a slim wrapper over
the same /auth/me endpoint useAuthStore already consumes. The merged
store gains the full set of additions Bert specified for B2a:

State:
- availableContexts / defaultContext (from /auth/me contexts block)
- lastContext (localStorage-persisted)
- portalToken (in-memory only, for the bearer-axios flavour)

Getters: isPortalUser, isOrganizerUser, isPlatformAdmin (alias of
isSuperAdmin), showContextSwitcher, hasRole(), hasAnyRole().

Actions: login(), verifyMfa() — both return typed discriminated
unions so login.vue (Phase H) consumes results without branching on
raw API response shapes. setLastContext, setPortalToken,
resolveLandingRoute, clearAll. clearAll dynamically imports
usePortalStore.reset() to clear portal sessionStorage on session-end —
this is the canonical session-cleanup hub now that the merge has
happened.

5 source files migrated from usePortalAuthStore → useAuthStore. The
PortalLayout.spec.ts mock follows. The boundaries matrix gains a
single new edge (`stores → stores-portal`) replacing the deleted
stores-portal/usePortalAuthStore which previously owned that
cross-zone call.

Adds 16 vitest specs in src/stores/__tests__/useAuthStore.spec.ts
covering setUser context hydration, hasRole/hasAnyRole, lastContext
localStorage persistence, resolveLandingRoute precedence
(portal/organizer/super_admin/multi-role/forceContext/forbidden
fallback), portalToken state, and clearAll cleanup.

Test count 162 → 178 (16 new). Frontend lint + typecheck clean.

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

apps/app/src/components/portal/UserAvatarMenu.vue

@@ -1,7 +1,7 @@
 <script setup lang="ts">
-import { usePortalAuthStore } from '@/stores/portal/usePortalAuthStore'
+import { useAuthStore } from '@/stores/useAuthStore'
 
-const authStore = usePortalAuthStore()
+const authStore = useAuthStore()
 const router = useRouter()
 
 const userInitials = computed(() => {