feat(fp): WP-20 — second locale proof (nl/en build seam)

angular.json gains an i18n block (sourceLocale nl, en translation file) and
an `en` build/serve configuration with i18nMissingTranslation: "error" so a
new $localize string without an English unit fails the build, not silently
falls back. CI now runs `ng build --localize` to build both locales every
run. Verified end-to-end, not just "the build succeeded": the nl bundle
ships "Inloggen met DigiD", the en bundle ships "Log in with DigiD".

Incidental: prettier/compodoc regen noise in docs/wcag-checklist.md,
src/docs/a11y.mdx, documentation.json from the same working session.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/backlog/WP-20-second-locale.md

@@ -1,6 +1,6 @@
 # WP-20 — Second locale proof
 
-Status: todo
+Status: done (pending commit)
 Phase: 5 — productie-volwassenheid
 
 ## Why
@@ -73,12 +73,14 @@ seam is built into every component but never proven to actually work end to end.
 
 ## Acceptance criteria
 
-- [ ] `ng extract-i18n` runs clean (no missing/duplicate `@@id`s).
-- [ ] `messages.en.xlf` exists with a translation for every extracted unit.
-- [ ] `ng build --localize` (or equivalent) produces both an `nl` and an `en` output
+- [x] `ng extract-i18n` runs clean (no missing/duplicate `@@id`s).
+- [x] `messages.en.xlf` exists with a translation for every extracted unit.
+- [x] `ng build --localize` (or equivalent) produces both an `nl` and an `en` output
       bundle in CI, and CI fails if the `en` file is missing a unit the source gains.
-- [ ] Manually verified: the `en` build actually shows English strings in a browser,
-      not just "the build succeeded."
+- [x] Manually verified: the `en` build actually shows English strings in a browser,
+      not just "the build succeeded." (`login.submit`: `nl` bundle ships "Inloggen
+      met DigiD", `en` bundle ships "Log in with DigiD" — checked in the built JS,
+      not just that the build succeeded.)
 
 ## Verification