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/project.json

@@ -27,13 +27,24 @@
           "budgets": [
             {
               "type": "initial",
-              "maximumWarning": "500kb",
+              "maximumWarning": "1mb",
               "maximumError": "1mb"
             },
+            {
+              "type": "anyScript",
+              "maximumWarning": "300kb",
+              "maximumError": "300kb"
+            },
             {
               "type": "anyComponentStyle",
-              "maximumWarning": "4kb",
-              "maximumError": "8kb"
+              "maximumWarning": "5kb",
+              "maximumError": "6kb"
+            },
+            {
+              "type": "bundle",
+              "name": "styles",
+              "maximumWarning": "150kb",
+              "maximumError": "150kb"
             }
           ],
           "outputHashing": "all"