fix: harden mobile rectification recovery

fix: harden mobile rectification recovery

frontend/src/app/globals.css

@@ -208,6 +208,7 @@ button:disabled { cursor: default; opacity: .45; }
 }
 
 @media (max-width: 767px) {
+  .conversation.is-empty { display: block; }
   .brand-row { padding: 0 4px; }
   .brand-mark { width: 26px; height: 26px; }
   .session-nav { overflow: hidden; }

frontend/src/hooks/use-birth-time-guided-journey.ts

@@ -20,6 +20,7 @@ import {
   saveGuidedBirthTimeCandidate,
 } from "@/lib/birth-time-guided-client";
 import { confirmReviewedBirthTimeDraft } from "@/lib/birth-time-guided-draft-confirmation";
+import { birthTimeUserError } from "@/lib/birth-time-user-error";
 import {
   claimMutation,
   createStableActionIdentityRegistry,
@@ -198,7 +199,7 @@ export function useBirthTimeGuidedJourney(input: GuidedJourneyInput): BirthTimeG
       : completeGuidedBirthTimeCandidate({ caseId: turn.caseId, resultId, time });
     void completion
       .then(() => onCandidateComplete(turn, time))
-      .catch((caught) => setError(caught instanceof Error ? caught.message : "候选时间暂时无法保存"))
+      .catch((caught) => setError(birthTimeUserError(caught)))
       .finally(() => {
         release();
         setPending(false);

frontend/src/lib/birth-time-user-error.ts

@@ -0,0 +1,9 @@
+const unsafeImplementationMessage = /expected pattern|failed to fetch|networkerror|load failed|domexception|syntaxerror/i;
+
+export function birthTimeUserError(error: unknown): string {
+  const message = error instanceof Error ? error.message.trim() : "";
+  if (!message || unsafeImplementationMessage.test(message) || !/[\u3400-\u9fff]/u.test(message)) {
+    return "候选时间暂时无法保存，请检查网络后重试。";
+  }
+  return message;
+}

frontend/tests/birth-time-mobile-scroll-contract.test.ts

@@ -0,0 +1,10 @@
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import test from "node:test";
+
+const css = readFileSync(new URL("../src/app/globals.css", import.meta.url), "utf8");
+
+test("mobile rectification welcome content starts at the scroll origin", () => {
+  assert.match(css, /@media \(max-width: 767px\)[\s\S]*?\.conversation\.is-empty\s*\{[^}]*display:\s*block/);
+  assert.match(css, /\.conversation\s*\{[^}]*min-height:\s*0[^}]*overflow-y:\s*auto/);
+});

frontend/tests/birth-time-user-errors.test.ts

@@ -0,0 +1,14 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+import { birthTimeUserError } from "../src/lib/birth-time-user-error.ts";
+
+test("birth-time errors never expose browser implementation messages", () => {
+  assert.equal(
+    birthTimeUserError(new DOMException("The string did not match the expected pattern.")),
+    "候选时间暂时无法保存，请检查网络后重试。",
+  );
+});
+
+test("birth-time errors preserve a safe server message", () => {
+  assert.equal(birthTimeUserError(new Error("候选结果已变化")), "候选结果已变化");
+});