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>

angular.json

@@ -17,6 +17,14 @@
       "root": "",
       "sourceRoot": "src",
       "prefix": "app",
+      "i18n": {
+        "sourceLocale": "nl",
+        "locales": {
+          "en": {
+            "translation": "src/locale/messages.en.xlf"
+          }
+        }
+      },
       "architect": {
         "build": {
           "builder": "@angular/build:application",
@@ -31,7 +39,8 @@
               }
             ],
             "styles": ["src/styles.scss"],
-            "polyfills": ["@angular/localize/init"]
+            "polyfills": ["@angular/localize/init"],
+            "i18nMissingTranslation": "error"
           },
           "configurations": {
             "production": {
@@ -59,6 +68,9 @@
               "optimization": false,
               "extractLicenses": false,
               "sourceMap": true
+            },
+            "en": {
+              "localize": ["en"]
             }
           },
           "defaultConfiguration": "production"
@@ -71,6 +83,9 @@
             },
             "development": {
               "buildTarget": "atomic-design-poc:build:development"
+            },
+            "en": {
+              "buildTarget": "atomic-design-poc:build:development,en"
             }
           },
           "defaultConfiguration": "development"