feat: add birth date value adapter

frontend/src/lib/birth-time-intake-model.ts

@@ -1,5 +1,8 @@
+import { format, isValid, parse } from "date-fns";
 import type { JourneySnapshot } from "./birth-time-journey.ts";
 
+const birthDatePattern = "yyyy-MM-dd";
+
 export type BirthTimeSource =
   | ""
   | "hospital_record"
@@ -39,6 +42,17 @@ export type BirthTimeDraft = {
 
 export type BirthTimeDraftPatch = Partial<BirthTimeDraft>;
 
+export function parseBirthDate(value: string): Date | undefined {
+  if (value === "") return undefined;
+  const parsed = parse(value, birthDatePattern, new Date(2000, 0, 1));
+  if (!isValid(parsed) || format(parsed, birthDatePattern) !== value) return undefined;
+  return parsed;
+}
+
+export function formatBirthDate(value: Date): string {
+  return format(value, birthDatePattern);
+}
+
 export const birthTimeSourceOptions = [
   { value: "hospital_record", label: "出生证明或医院记录", hint: "先检查前后两分钟是否稳定" },
   { value: "family_exact", label: "家人明确记得具体时间", hint: "进行 5—15 分钟轻量校正" },