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.

docs/api/openapi.yaml

@@ -25894,6 +25894,7 @@ components:
             - it
             - pl
             - ko
+            - nl
         practiceName:
           type: string
           maxLength: 120
@@ -28352,6 +28353,7 @@ components:
             - it
             - pl
             - ko
+            - nl
         today:
           description: "Optional override of the reference date used to resolve relative phrases (\"tomorrow\", \"next Monday\").
             Format: `YYYY-MM-DD`. Defaults to the account's own day in its time zone."
@@ -29302,6 +29304,7 @@ components:
             - it
             - pl
             - ko
+            - nl
           description: One of the interface locales this build ships.
       required:
         - locale
@@ -34591,6 +34594,7 @@ components:
             - it
             - pl
             - ko
+            - nl
         timezone:
           type: string
           minLength: 1
@@ -34645,6 +34649,7 @@ components:
             - it
             - pl
             - ko
+            - nl
         timezone:
           type: string
           minLength: 1
@@ -34862,6 +34867,7 @@ components:
                 - it
                 - pl
                 - ko
+                - nl
             - type: "null"
         timezone:
           type: string
@@ -55789,6 +55795,7 @@ components:
                 - it
                 - pl
                 - ko
+                - nl
           required:
             - locale
           additionalProperties: false
@@ -82735,6 +82742,7 @@ components:
                 - it
                 - pl
                 - ko
+                - nl
             - type: "null"
         timezone:
           type: string