fix(ui): keep mobile onboarding and login usable

Hide the dead composer during birth-time intake, replace internal
copy with user-facing hints, and keep short-screen login, location
search, and session history reachable on phones.

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

@@ -83,8 +83,8 @@ export function formatBirthDate(value: Date): string {
 }
 
 export const birthTimeSourceOptions = [
-  { value: "family_exact", label: "我知道准确出生时间", hint: "保存为初始化填报时间，不会自动标记为引擎确认" },
-  { value: "period_only", label: "我不确定准确时间", hint: "告诉我们大致时段；完全不清楚也可以直接跳过" },
+  { value: "family_exact", label: "我知道准确出生时间", hint: "按医院记录或家人记得的时间填写，精确到分钟" },
+  { value: "period_only", label: "我不确定准确时间", hint: "只知道大概几点、上午下午，或完全不清楚" },
 ] as const;
 
 export const birthTimeSourceDefaults = {
@@ -223,6 +223,15 @@ export function isBirthTimeDraftReady(draft: BirthTimeDraft) {
   }
 }
 
+export function birthTimeDraftReadyHint(draft: BirthTimeDraft) {
+  if (!parseBirthDate(draft.date)) return "请先选择出生日期";
+  if (!draft.birthTimeSource) return "请选择你对出生时间了解多少";
+  if (isBirthTimeDraftReady(draft)) return "";
+  if (draft.birthTimeSource === "period_only") return "请选择一个大致时段";
+  if (draft.birthTimeSource === "unknown") return "";
+  return "请填写时和分";
+}
+
 /**
  * Whether the user has finished declaring what they actually know about birth time.
  * This is an onboarding condition, not a claim that an exact chart minute is ready.