feat(portal-shell): collapse accessibility routes into one i18n-marked route (#94)

## Summary

Continue ADR-0019: replace the `/accessibility` + `/accessibilite` twin routes with a single canonical route whose content is i18n-marked in the template. The per-locale build (en/fr) already inlines the right copy — the route-data + `copy()` service indirection is no longer carrying its weight.

## What changes

- **`AccessibilityStatement`** loses its `ActivatedRoute` injection, the `Lang` discriminator, and the `COPY` lookup table. The component is now a plain shell over the template.
- **`accessibility.html`** carries the title + intro + status panel as `i18n="@@page.accessibility.*"` markers. Six new trans-units in [`messages.fr.xlf`](apps/portal-shell/src/locale/messages.fr.xlf) provide the French copy — verbatim from the old `COPY.fr` block, so the page reads the same in FR as before.
- **`app.routes.ts`** declares the single canonical route at `path: 'accessibility'` and keeps `/accessibilite` alive as a `redirectTo: 'accessibility'`. Drops `data: { lang: ... }` — no longer consumed.
- **`footer.html`** collapses the dual link into one i18n-marked link (`@@footer.accessibilityLink`). EN bundle reads "Accessibility statement"; FR bundle reads "Déclaration d'accessibilité".

## Decision worth flagging

The path stays in English across both locales for now: `/en/accessibility` and `/fr/accessibility`. Translating route *segments* (`/fr/declaration-d-accessibilite`) needs either a custom URL serializer or per-locale route trees — not worth the complexity at this scale. The page title and the link label already differ per locale via i18n, which is what's actually visible to users.

The historical `/accessibilite` path keeps working via the route-level redirect. Drops out of the codebase once analytics confirm no traffic reaches it.

## Test plan

- [x] `pnpm exec nx run-many -t lint test build --projects=portal-shell` — green. **35 / 35 specs** (was 36; the obsolete `lang`-fallback test is removed).
- [x] Production build emits both locales. FR bundle contains `Statut`, `Déclaration`, the FR intro / panel bodies. No leftover English on swept strings.
- [x] `extract-i18n` clean (49 unique units now: +5 for `page.accessibility.*` + `footer.accessibilityLink`, −0; the old route-data `lang` markers were not i18n).
- [ ] Manual: serve-static then `/en/accessibility` and `/fr/accessibility` render their respective content; `/fr/accessibilite` 301-redirects to `/fr/accessibility`.
- [ ] Manual: footer shows one link, locale-aware ("Accessibility statement" / "Déclaration d'accessibilité").
- [ ] Manual: browser tab title flips between bundles ("Accessibility statement · APF Portal" / "Déclaration d'accessibilité · Portail APF").

## What this PR explicitly does NOT do

- Translate the URL path segment (next ADR-only refinement if needed).
- Add the locale switcher in the footer — that's the next PR on the i18n track.
- Wire a CI gate on missing translations.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #94

apps/portal-shell/src/locale/messages.fr.xlf

@@ -26,6 +26,10 @@
         <source>Legal</source>
         <target>Mentions légales</target>
       </trans-unit>
+      <trans-unit id="footer.accessibilityLink" datatype="html">
+        <source>Accessibility statement</source>
+        <target>Déclaration d’accessibilité</target>
+      </trans-unit>
       <trans-unit id="footer.copyright" datatype="html">
         <source>© <x id="INTERPOLATION" equiv-text="{{ year }}"/> APF France handicap</source>
         <target>© <x id="INTERPOLATION" equiv-text="{{ year }}"/> APF France handicap</target>
@@ -163,6 +167,28 @@
         <target>Portail APF</target>
       </trans-unit>
 
+      <!-- page.accessibility -->
+      <trans-unit id="page.accessibility.intro.body" datatype="html">
+        <source> The APF Portal is committed to making its digital services accessible, in line with WCAG 2.2 level AA and the French RGAA 4.1 reference, with targeted AAA criteria on the items most impactful to the association&apos;s user base (see ADR-0016). </source>
+        <target> Le portail APF s’engage à rendre ses services numériques accessibles, conformément au RGAA 4.1 et à la norme WCAG 2.2 niveau AA, avec des critères AAA ciblés sur les points à fort impact pour les usagers de l’association (voir ADR-0016). </target>
+      </trans-unit>
+      <trans-unit id="page.accessibility.intro.title" datatype="html">
+        <source> About this statement </source>
+        <target> À propos de cette déclaration </target>
+      </trans-unit>
+      <trans-unit id="page.accessibility.panel.body" datatype="html">
+        <source> This statement is awaiting review and sign-off by the APF France handicap user panel. The final version will integrate the RGAA audit grid, the list of any identified gaps, the remediation plan, and the contact details for reporting accessibility issues. </source>
+        <target> Cette déclaration est en attente de revue et de validation par le panel d’usagers d’APF France handicap. La version définitive intégrera la grille d’audit RGAA, le recensement des écarts éventuels, le plan de mise en conformité, et les coordonnées de signalement. </target>
+      </trans-unit>
+      <trans-unit id="page.accessibility.panel.title" datatype="html">
+        <source> Status of this document </source>
+        <target> Statut du présent document </target>
+      </trans-unit>
+      <trans-unit id="page.accessibility.title" datatype="html">
+        <source> Accessibility statement </source>
+        <target> Déclaration d’accessibilité </target>
+      </trans-unit>
+
       <!-- page.home -->
       <trans-unit id="page.home.intro" datatype="html">
         <source> This is the placeholder home page for the APF Portal. Real content lands once the comms team and the accessibility review panel sign off on the copy. </source>