feat(onboarding): pick the birth place by level instead of by search

The single search field asked people to type a place name and then judge
which of several near-identical results was theirs, which is the one thing
they cannot verify about their own birth record. Province, city and district
are now chosen from the dataset the app already ships, so there is nothing to
type and nothing to disambiguate.

Levels that offer no choice collapse: municipalities show one level, and
prefecture cities without districts stop at the city. A district can be left
as the city centre, which is accurate enough because the chart only needs
coordinates and a timezone.

The timezone is resolved once for the chosen place rather than on every
keystroke, through a dedicated route that both this picker and the Geoapify
path share.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/app/api/locations/timezone/route.ts

@@ -0,0 +1,28 @@
+import { NextResponse } from "next/server";
+import { resolveBirthLocationTimezone } from "@/lib/birth-location-timezone-service";
+import { birthLocationTimezoneQuerySchema } from "@/lib/location-contract";
+import { createServerSupabaseClient } from "@/lib/supabase/server";
+
+export const runtime = "nodejs";
+
+export async function POST(request: Request) {
+  const supabase = await createServerSupabaseClient();
+  const { data: { user }, error: authError } = await supabase.auth.getUser();
+  if (authError || !user) return NextResponse.json({ error: "请先登录" }, { status: 401 });
+
+  let body: unknown;
+  try {
+    body = await request.json();
+  } catch {
+    return NextResponse.json({ error: "时区解析参数不正确" }, { status: 400 });
+  }
+
+  const parsed = birthLocationTimezoneQuerySchema.safeParse(body);
+  if (!parsed.success) return NextResponse.json({
+    error: "时区解析参数不正确",
+    details: parsed.error.flatten(),
+  }, { status: 400 });
+
+  const result = await resolveBirthLocationTimezone(parsed.data);
+  return NextResponse.json(result, { status: result.status === "ok" ? 200 : 503 });
+}