refactor(apps/app): decouple axios from impersonation sessionStorage contract

Chose Option A from the follow-up brief: useImpersonationStore
already holds an `ImpersonationState` ref hydrated from
sessionStorage at store-init and exposes the active impersonation
target user as a public `targetUserId` computed. The store is the
canonical source; sessionStorage is just its persistence sidecar.

Adds a fifth callback `getImpersonationTargetUserId: () => string
| null` to AxiosBindingsDeps and replaces the
sessionStorage.getItem('crewli_impersonation') + JSON.parse block
in the request interceptor with a single `deps.getImpersonationTargetUserId()`
call. The bindings plugin wires it to
`useImpersonationStore().targetUserId`.

After this commit lib/axios.ts has zero references to
sessionStorage and zero magic strings about impersonation
persistence — the only persistence-mechanism knowledge left is in
useImpersonationStore (where it belongs) and in
plugins/3.axios-bindings.ts (allowed to know about stores). The
HTTP module is now unambiguously pure infrastructure.

Behavior preserved 1:1: the store hydrates from sessionStorage
synchronously inside the defineStore factory, so the very first
HTTP request after page load sees the same target user id as
before.

Co-Authored-By: Claude <noreply@anthropic.com>

apps/app/src/plugins/3.axios-bindings.ts

@@ -12,6 +12,7 @@ import { useOrganisationStore } from '@/stores/useOrganisationStore'
 export default function (_: App): void {
   registerInterceptors(apiClient, {
     getActiveOrgId: () => useOrganisationStore().activeOrganisationId,
+    getImpersonationTargetUserId: () => useImpersonationStore().targetUserId,
     notify: (message, level) => useNotificationStore().show(message, level),
     onAuthFail: () => {
       const authStore = useAuthStore()