feat: replace native birth date input

frontend/DESIGN.md

@@ -102,6 +102,14 @@ The base unit is 4px. Tokens are `--space-1: 4px`, `--space-2: 8px`, `--space-3:
 - **Accessibility:** native radio inputs remain focusable, every conditional field has a persistent label, status text uses live regions, and the complete flow is keyboard operable.
 - **Motion:** source-dependent fields enter with the existing 180ms opacity/vertical reveal; reduced-motion removes the translation.
 
+### Birth date picker
+
+- **Composition:** shadcn outline Button trigger, Base UI Popover, and a single-select React DayPicker Calendar.
+- **Range:** local dates from 1900-01-01 through today; future dates are disabled. Month and year dropdowns provide direct navigation, with newest years first.
+- **Value:** display Chinese long dates while emitting the existing `YYYY-MM-DD` profile value without UTC conversion.
+- **States:** empty, open, selected, focus-visible, disabled confirmed profile, and unavailable date.
+- **Accessibility:** visible label, explicit trigger naming, 44px targets, keyboard calendar navigation, focus return, and collision-safe popup positioning.
+
 ### Model selector
 
 - **Structure:** a compact text trigger sits below the composer and opens an upward popover aligned to its left edge. The trigger shows only the active model name; each option shows only its model name and radio selection state.

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

@@ -0,0 +1,72 @@
+"use client";
+
+import { format } from "date-fns";
+import { zhCN } from "date-fns/locale";
+import { CalendarIcon } from "lucide-react";
+import { useId, useState } from "react";
+
+import { Button } from "@/components/ui/button";
+import { Calendar } from "@/components/ui/calendar";
+import { Popover, PopoverContent, PopoverTrigger } from "@/components/ui/popover";
+import { formatBirthDate, parseBirthDate } from "@/lib/birth-time-intake-model";
+
+type BirthDatePickerProps = {
+  readonly value: string;
+  readonly disabled: boolean;
+  readonly onChange: (value: string) => void;
+};
+
+export function BirthDatePicker({ value, disabled, onChange }: BirthDatePickerProps) {
+  const labelId = useId();
+  const valueId = useId();
+  const [open, setOpen] = useState(false);
+  const selected = parseBirthDate(value);
+  const today = new Date();
+  today.setHours(0, 0, 0, 0);
+
+  return (
+    <div className="grid gap-2">
+      <span id={labelId}>出生日期</span>
+      <Popover open={open} onOpenChange={setOpen}>
+        <PopoverTrigger
+          render={<Button
+            type="button"
+            variant="outline"
+            disabled={disabled}
+            aria-labelledby={`${labelId} ${valueId}`}
+            data-empty={selected === undefined}
+            className="w-full justify-start px-3 text-left font-normal data-[empty=true]:text-muted-foreground"
+          />}
+        >
+          <CalendarIcon aria-hidden="true" />
+          <span id={valueId}>
+            {selected === undefined
+              ? "选择出生日期"
+              : format(selected, "PPP", { locale: zhCN })}
+          </span>
+        </PopoverTrigger>
+        <PopoverContent align="start" className="w-auto p-0">
+          <Calendar
+            key={value || "empty"}
+            mode="single"
+            className="[--cell-size:2.75rem] [&_button[data-selected-single=true]]:text-primary-foreground!"
+            locale={zhCN}
+            selected={selected}
+            defaultMonth={selected ?? today}
+            captionLayout="dropdown"
+            navLayout="after"
+            startMonth={new Date(1900, 0)}
+            endMonth={today}
+            reverseYears
+            disabled={{ before: new Date(1900, 0, 1), after: today }}
+            onSelect={(nextDate) => {
+              if (nextDate === undefined) return;
+              onChange(formatBirthDate(nextDate));
+              setOpen(false);
+            }}
+          />
+        </PopoverContent>
+      </Popover>
+    </div>
+  );
+}

frontend/src/components/birth-time-intake.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import { useId } from "react";
+import { BirthDatePicker } from "@/components/birth-date-picker";
 import {
   birthTimePeriodOptions,
   birthTimeSourceOptions,
@@ -58,16 +59,11 @@ export function BirthTimeIntakeFields({ value, onPatch }: BirthTimeIntakeProps)
 
   return (
     <div className="birth-time-intake">
-      <label>
-        <span>出生日期</span>
-        <input
-          required
-          disabled={isConfirmed}
-          type="date"
-          value={value.date}
-          onChange={(event) => onPatch({ date: event.target.value })}
-        />
-      </label>
+      <BirthDatePicker
+        value={value.date}
+        disabled={isConfirmed}
+        onChange={(date) => onPatch({ date })}
+      />
 
       {source === "legacy_import" && (
         <p className="birth-time-legacy-note">

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

@@ -3,9 +3,24 @@ import { existsSync, readFileSync } from "node:fs"
 import test from "node:test"
 
 const packageJson = readFileSync(new URL("../package.json", import.meta.url), "utf8")
+const intake = readFileSync(new URL("../src/components/birth-time-intake.tsx", import.meta.url), "utf8")
+const pickerUrl = new URL("../src/components/birth-date-picker.tsx", import.meta.url)
 
 test("provides the shadcn calendar and popover primitives", () => {
   assert.equal(existsSync(new URL("../src/components/ui/calendar.tsx", import.meta.url)), true)
   assert.equal(existsSync(new URL("../src/components/ui/popover.tsx", import.meta.url)), true)
   assert.match(packageJson, /"react-day-picker"/)
 })
+
+test("replaces the native birth date input with the shadcn date picker", () => {
+  assert.equal(existsSync(pickerUrl), true)
+  assert.doesNotMatch(intake, /type="date"/)
+  assert.match(intake, /<BirthDatePicker/)
+  const picker = readFileSync(pickerUrl, "utf8")
+  assert.match(picker, /<PopoverTrigger/)
+  assert.match(picker, /render=\{<Button/)
+  assert.match(picker, /<Calendar/)
+  assert.match(picker, /captionLayout="dropdown"/)
+  assert.match(picker, /startMonth=\{new Date\(1900, 0\)\}/)
+  assert.match(picker, /reverseYears/)
+})