fix(chat): fold natal reports and drop homepage topic cards

Homepage no longer waits on /api/onboarding for six starter questions.
Natal answers show the spoken layer first; the Level 2 skeleton sits in a
collapsed 完整分析 block. Wide tables scroll sideways on a phone. Form
inputs are 16px so iOS does not zoom on focus.

frontend/src/lib/chat-answer-split.ts

@@ -0,0 +1,60 @@
+/**
+ * Split a chat answer into the spoken layer (before the first H2) and the
+ * report skeleton (from that H2 onward). The cut is any ATX H2, so a drifted
+ * first heading still folds instead of dumping the whole body.
+ *
+ * Fenced `##` lines are not cuts. The fence rule matches `chat-markdown-split`.
+ */
+
+export type SpokenAndReportSplit = Readonly<{
+  spoken: string;
+  report: string;
+  headings: readonly string[];
+}>;
+
+const FENCE = /^\s{0,3}(`{3,}|~{3,})/;
+const H2 = /^##\s+\S/;
+
+function extractHeadings(report: string): string[] {
+  const lines = report.split("\n");
+  let insideFence = false;
+  const headings: string[] = [];
+  for (const line of lines) {
+    if (FENCE.test(line)) insideFence = !insideFence;
+    if (!insideFence && H2.test(line)) {
+      headings.push(line.replace(/^##\s+/, "").trim());
+    }
+  }
+  return headings;
+}
+
+export function splitSpokenAndReport(text: string): SpokenAndReportSplit {
+  const lines = text.split("\n");
+  let insideFence = false;
+  let cut = -1;
+  for (let index = 0; index < lines.length; index += 1) {
+    const line = lines[index] ?? "";
+    if (FENCE.test(line)) insideFence = !insideFence;
+    if (!insideFence && H2.test(line)) {
+      cut = index;
+      break;
+    }
+  }
+  if (cut < 0) return { spoken: text, report: "", headings: [] };
+  const spoken = lines.slice(0, cut).join("\n").replace(/\s+$/u, "");
+  const report = lines.slice(cut).join("\n");
+  return { spoken, report, headings: extractHeadings(report) };
+}
+
+export function shortenAnswerHeading(heading: string): string {
+  if (heading === "统一参数与原始结构") return "统一参数";
+  if (heading === "技法审计表") return "技法审计";
+  return heading;
+}
+
+export function answerDetailCaption(headings: readonly string[]): string {
+  if (headings.length === 0) return "";
+  const labels = headings.slice(0, 3).map(shortenAnswerHeading);
+  if (headings.length <= 3) return labels.join("、");
+  return `${labels.join("、")} 等 ${headings.length} 节`;
+}