feat(i18n): add Dutch as an eighth end-to-end locale

Adds messages/nl.json (AI-initial, informal je/jij register, full key
parity with en.json) and registers nl everywhere the locale list lives:
config, loaders, Intl/date-fns mapping, zod enums, OpenAPI spec, scripts,
the managed-profile picker and the notification locale guards.

Dutch follows the Korean pattern for the AI surfaces (English body plus a
Dutch reply-language footer, no safety-contract matrix) and gets its own
banks in the outbound safety screen (dose, risk, causal, continuation),
the prose-grounding and citation-coverage tables, the reference-range
prose and the no-key fallback copy, each with Dutch test fixtures.

PHQ-9 / GAD-7 / WHO-5 are not marked validated for nl yet: the Dutch
wording has not been read against the published forms, so Dutch readers
get the English items plus the existing note until it has.

scripts/split-description.mjs

@@ -18,7 +18,7 @@
  */
 import { readFileSync, writeFileSync } from "node:fs";
 
-const LOCALES = ["de", "en", "es", "fr", "it", "pl", "ko"];
+const LOCALES = ["de", "en", "es", "fr", "it", "pl", "ko", "nl"];
 const BOUNDARY = /(?<=[.!?])\s+(?=[A-ZÄÖÜÀÂÉÈÊÎÔÙÛÇŁŚŹŻĄĆĘŃÓ0-9„"„«])/u;
 
 /**