feat(portal-shell): replace nx-welcome with real shell and home + budgets per ADR-0017

Cuts the Nx scaffold's `NxWelcome` page (938 LOC of placeholder
markup) and lays down the actual portal layout — header, main,
footer landmarks plus a skip link — with a real home page and
the two RGAA-mandated accessibility statement routes. Bundle
budgets in `project.json` are tightened to match ADR-0017.

Layout shell (apps/portal-shell/src/app/):

- components/header/  — brand link + primary nav landmark stub
- components/footer/  — accessibility statement links (FR + EN)
                        + version badge
- app.html / app.scss — flex column with sticky footer + skip
                        link to #main-content (WCAG 2.4.1)

Pages:

- pages/home/         — welcome heading + "system status" widget
                        that fetches /api/health from the BFF.
                        Doubles as a smoke test of the full
                        SPA → BFF stack: CORS, OTel trace
                        propagation, Pino log correlation.
- pages/accessibility/ — single component, content selected by
                        route data (`lang: 'fr' | 'en'`). Mounted
                        at /accessibility (en) and /accessibilite
                        (fr). v1 carries placeholder copy
                        explicitly framed as "awaiting APF user
                        panel review" — RGAA + ADR-0016 require
                        the routes to exist; the substantive
                        statement (audit grid, gaps list,
                        remediation plan) lands separately.

Routing & wiring:

- app.routes.ts adds the three routes, all lazy-loaded.
- app.config.ts adds `provideHttpClient(withFetch())` so
  HttpClient delegates to the patched browser `fetch` and the
  W3C `traceparent` header propagates automatically (ADR-0012
  phase 2).
- nx-welcome.ts removed; app.ts no longer imports it.

Bundle budgets (apps/portal-shell/project.json):

- initial            ≤ 1 MB raw  (≈ 300 KB gzip target per ADR)
- anyScript          ≤ 300 KB raw (≈ 100 KB gzip per lazy chunk)
- anyComponentStyle  ≤ 6 KB raw  (5 KB warning floor)
- bundle "styles"    ≤ 150 KB raw

`maximumWarning == maximumError` so an overshoot fails the build
(matching `type: "error"` in spirit). Angular CLI compares raw
sizes; ADR-0017 §Confirmation is updated to record the gzip→raw
translation and the deferred follow-up that will add a CI check
on the actual gzipped transfer size (Angular has no native
gzip-mode budget).

Verified locally:
- `nx run-many -t lint test build` → 8 projects green, 12 tests
  pass for portal-shell across 5 specs.
- `nx build portal-shell --configuration=production` → initial
  bundle 326.99 KB raw / 89.82 KB transfer (gzip) — well under
  the ADR-0017 300 KB gzip target. Lazy chunks: home 2.66 KB,
  accessibility 2.90 KB.

Out of scope (separate PRs):

- Real RGAA audit content (APF user-panel review).
- Design tokens system in libs/shared/tokens.
- Reusable UI primitives in libs/shared/ui.
- User-preferences panel (ADR-0016 §"User-preferences panel").
- i18n machinery (@angular/localize) — for v1 the FR/EN copy is
  inlined and route-driven; real i18n is a separate ADR.
- env-config (Angular environment.ts) — DATABASE-of-the-front
  hard-coded URL `http://localhost:3000/api/health` will move
  there alongside the OTLP endpoint when the env-config PR
  lands.
- CI gzip-transfer-size assertion to complement the raw-size
  Angular budgets.

apps/portal-shell/src/app/app.scss

@@ -0,0 +1,32 @@
+// Page-level layout: header + main + footer in a flex column so the
+// footer sticks to the viewport bottom even when the main content is
+// short. Driven on the host so it covers the whole document.
+:host {
+  display: flex;
+  flex-direction: column;
+  min-height: 100vh;
+}
+
+// Skip-link (WCAG 2.4.1 "Bypass Blocks"). Hidden by default, fully
+// visible and focusable when reached via Tab from the address bar.
+// Plain CSS rather than the `sr-only` Tailwind utilities so the
+// visual state on focus is as predictable as possible across themes.
+.skip-link {
+  position: absolute;
+  top: -40px;
+  left: 0.75rem;
+  z-index: 50;
+  padding: 0.5rem 0.75rem;
+  background: #1d4ed8;
+  color: #fff;
+  border-radius: 0.25rem;
+  text-decoration: none;
+  transition: top 0.15s ease-out;
+
+  &:focus,
+  &:focus-visible {
+    top: 0.5rem;
+    outline: 2px solid #93c5fd;
+    outline-offset: 2px;
+  }
+}