fix(auth): make admin pages reachable — async capability guard + sticky dev role + nav

The admin pages (/beheer/stamdata, /brief/huisstijl) were unreachable in the browser,
for three compounding reasons — all fixed here:

- **Guard raced /me.** capabilityGuard read can() synchronously while /me was still
  loading, so it denied even an entitled admin (deny-by-default) and bounced to /login.
  It's now async: awaits AccessStore.whenReady() (new — resolves once /me settles), then
  allows if entitled; an authenticated-but-unentitled user goes to /dashboard, anonymous
  to /login. + auth.guard.spec (the missing test that let this ship).
- **Dev role wasn't sticky.** currentRole() read ?role= from the URL on every request,
  but login/nav drop the param, silently reverting admin→drafter mid-session and 403-ing
  the admin endpoints. It now persists the role per-tab (sessionStorage), so every
  role-aware request keeps it. Dev-only (the interceptor is wired only under isDevMode).
- **No way in.** Added capability-gated Huisstijl + Stamdata links to the header (shown
  only when /me grants the cap); injecting AccessStore there also warms /me early. New
  en translations for the two labels; site-header story stubs AccessStore (+ AsAdmin
  variant) so it needs no HTTP.

Verified live: with ?role=admin the header shows both links, clicking Stamdata loads the
grid (GET /api/v1/stamdata → 200, was 403→redirect); a non-admin sees no link. Full
`npm run ci` green (310 tests); site-header stories pass axe.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/shared/application/access.store.ts

@@ -1,4 +1,6 @@
 import { Injectable, computed, inject } from '@angular/core';
+import { toObservable } from '@angular/core/rxjs-interop';
+import { filter, firstValueFrom } from 'rxjs';
 import { RemoteData, fromResource } from '@shared/application/remote-data';
 import { Capability } from '@shared/domain/capability';
 import { MeAdapter, parseMe } from '@shared/infrastructure/me.adapter';
@@ -40,4 +42,13 @@ export class AccessStore {
     const tag = this.capabilities().tag;
     return tag === 'Success' || tag === 'Failure';
   });
+
+  private ready$ = toObservable(this.ready);
+  /** Resolves once `/me` has settled (success or failure). The `capabilityGuard` awaits
+      this before deciding — otherwise it reads `can()` while `/me` is still loading and
+      wrongly denies (deny-by-default), bouncing even an entitled user. */
+  async whenReady(): Promise<void> {
+    if (this.ready()) return;
+    await firstValueFrom(this.ready$.pipe(filter((r) => r)));
+  }
 }