fix: default birth picker to 1997

frontend/src/components/birth-date-picker.tsx

@@ -17,6 +17,8 @@ type BirthDatePickerProps = {
   readonly onChange: (value: string) => void;
 };
 
+const emptyDefaultMonth = new Date(1997, 0, 1);
+
 export function BirthDatePicker({ value, disabled, onChange }: BirthDatePickerProps) {
   const labelId = useId();
   const valueId = useId();
@@ -53,7 +55,7 @@ export function BirthDatePicker({ value, disabled, onChange }: BirthDatePickerPr
             className="[--cell-size:2.75rem] [&_button[data-selected-single=true]]:text-primary-foreground!"
             locale={zhCN}
             selected={selected}
-            defaultMonth={selected ?? today}
+            defaultMonth={selected ?? emptyDefaultMonth}
             captionLayout="dropdown"
             startMonth={new Date(1900, 0)}
             endMonth={today}

frontend/tests/birth-date-picker-contract.test.ts

@@ -33,3 +33,13 @@ test("keeps the shadcn date-of-birth dropdown navigation order", () => {
   // Then the picker must not move the full-width nav after the caption selects
   assert.doesNotMatch(picker, /navLayout="after"/)
 })
+
+test("opens an empty birth date picker at 1997 instead of the current year", () => {
+  // Given: a new user has not selected a birth date yet.
+  const picker = readFileSync(pickerUrl, "utf8")
+
+  // When: the popover chooses its initial visible month.
+  // Then: it starts in 1997 so common birth years are nearby.
+  assert.match(picker, /const emptyDefaultMonth = new Date\(1997, 0, 1\)/)
+  assert.match(picker, /defaultMonth=\{selected \?\? emptyDefaultMonth\}/)
+})