feat(portal-shell): wire SPA auth state to BFF /me + header login/logout widget (#113)

## Summary

First user-visible piece of the auth track. The portal-shell SPA now consumes the BFF auth surface (`/api/auth/me`, `/api/auth/login`, `/api/auth/logout`) and the header reflects sign-in state.

- `libs/feature/auth` ships an `AuthService` that fetches `/auth/me` on first injection, holds a signal-backed `AuthState` (`loading` / `anonymous` / `authenticated` / `error`) and exposes `currentUser` + `isLoading` computed signals plus `login()` / `logout()` / `refresh()` methods.
- The header's right-side widget renders four states: a sign-in button when anonymous, the user avatar (initials, `JD` for "Jane Doe") + display name + sign-out button when authenticated, a loading dot before `/me` resolves, and a "Can't reach the server" chip on non-401 failures.
- `login()` / `logout()` go through an injected `AUTH_NAVIGATOR` token whose default calls `window.location.assign(url)`. Specs override it with `vi.fn()` — no `window.location` mocking required.

## Notable choices

**Auto-bootstrap on construction, not via `provideAppInitializer`.** The service fires `/me` from its constructor (unawaited) so consuming components transition through the explicit `loading` state. Blocking app boot on the round-trip would push the first paint behind the network call — bad for TTFB, especially on slow links. The header handles `loading` as a first-class state.

**Discriminated `AuthState` over flat fields.** A single source of truth (`state()`) with four `kind`s lets templates `switch` and narrow automatically. `currentUser` and `isLoading` are computed conveniences but never out of sync with `state`.

**`AUTH_BFF_BASE_URL` + `AUTH_NAVIGATOR` injection tokens.** Decouples the lib from the host's `environment.ts` shape and keeps tests free of `window.location` redefinition (which jsdom resists across multiple specs in the same file — first redefine works, second throws "Cannot redefine property"). The host wires both in `app.config.ts`.

**Curated public user type.** `CurrentUser` mirrors the BFF's `/me` response (`oid`, `tid`, `username`, `displayName`) — no `amr`, no internal claims. The shape lives in `auth.types.ts` so feature code can import it without depending on Angular HTTP details.

**Distinct `error` state.** A network failure / 5xx surfaces a different UI than "not signed in" — "Can't reach the server" chip vs. sign-in button. Avoids the trap of treating any `/me` failure as "anonymous".

## Out of scope (next PRs)

- Route guards (protecting routes from anonymous users). For now the header is the only consumer.
- Auto-refresh of the session before idle timeout.
- HTTP interceptor that redirects to `/auth/login` on a 401 from any other BFF call.
- Per-locale styling polish on the new header strings.

## Test plan

- [x] `pnpm nx test feature-auth` → **8/8 pass**.
- [x] `pnpm nx test portal-shell` → **32/32 pass** (was 27 before).
- [x] `pnpm nx lint portal-shell feature-auth` → clean.
- [x] `pnpm nx build portal-shell` → main bundle 488 kB raw / 129.94 kB transfer; well under the 300 kB gzip budget from ADR-0017.
- [ ] Manual smoke once the BFF is up:
  - [ ] Anonymous landing → header shows "Sign in".
  - [ ] Click "Sign in" → BFF /login → Entra → callback → SPA lands with avatar + display name in the header.
  - [ ] Click "Sign out" → BFF /logout → Entra logout → back at SPA, header back to "Sign in".

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #113

apps/portal-shell/tsconfig.app.json

@@ -7,11 +7,14 @@
   "include": ["src/**/*.ts"],
   "exclude": ["src/**/*.spec.ts", "src/**/*.test.ts"],
   "references": [
+    {
+      "path": "../../libs/shared/state"
+    },
     {
       "path": "../../libs/shared/ui"
     },
     {
-      "path": "../../libs/shared/state"
+      "path": "../../libs/feature/auth"
     }
   ]
 }