feat(home): 首页开场语改成一行问候，占位符改「想聊什么都可以」

开场语从「称呼行 + 追问句」收成 claude.ai 式的单行问候：时段池、
星期池、回访池按顺序拼成一个数组，按 variantSelection 确定性取一条；
名字为空时跳过带称呼的条目。副标题行与 .starter-salutation 下线，
h1 降到 clamp(24px, 2.8vw, 30px) / line-height 1.25。追问改由输入框
占位符承担。

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JUei7K13cYxLHE3Axe4A45

frontend/src/components/starter-home.tsx

@@ -19,14 +19,15 @@ import type { RectificationEntrySummary } from "@/lib/rectification-entry";
  */
 
 export type StarterGreetingProps = {
-  readonly starterGreeting: { readonly salutation: string; readonly question: string };
+  readonly starterGreeting: { readonly text: string };
 };
 
+/* One line. The salutation/question pair it replaces asked the user's question
+   for them; the composer placeholder asks now. */
 export function StarterGreeting({ starterGreeting }: StarterGreetingProps) {
   return (
     <section className="starter-greeting-block" aria-labelledby="starter-heading">
-      <p className="starter-salutation">{starterGreeting.salutation}</p>
-      <h1 id="starter-heading">{starterGreeting.question}</h1>
+      <h1 id="starter-heading">{starterGreeting.text}</h1>
     </section>
   );
 }