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/app/(app)/page.tsx

@@ -721,7 +721,8 @@ export default function Home() {
   );
   const onboardingFormActive = !profileComplete && onboardingStep !== "name";
   const birthTimeContinueHint = onboardingStep === "birth" ? birthTimeDraftReadyHint(profileDraft) : "";
-  const starterGreeting = createStartGreetingParts(profile.name, new Date(), starterGreetingSelection);
+  const starterHasPriorSessions = sessions.some((session) => session.id !== activeSession?.id);
+  const starterGreeting = createStartGreetingParts(profile.name, new Date(), starterGreetingSelection, starterHasPriorSessions);
   const conversationAnchor = useConversationScrollAnchor(
     conversation,
     !rectificationSurfaceOpen && !starterHomeVisible,

frontend/src/app/globals.css

@@ -2089,29 +2089,25 @@ input:not([type="radio"]):not([type="checkbox"]):not([class^="ant-"]):not([class
   padding: var(--space-8) 0 var(--space-5);
 }
 
-/* The greeting: one line of salutation, one question. It used to be an h1 at
+/* The greeting: one line, two to five words. It used to be an h1 at
    clamp(34px, 5vw, 52px) inside a bordered hero card, with two 132px product
-   cards under it — together about 800px sitting on top of the composer. */
+   cards under it — together about 800px sitting on top of the composer; then a
+   salutation line above a question. The question moved into the composer
+   placeholder, so there is no second line to size against. */
 .starter-greeting-block {
   width: 100%;
   min-width: 0;
   text-align: center;
 }
 
-.starter-salutation {
-  margin: 0 0 var(--space-2);
-  color: var(--color-ink-tertiary);
-  font-size: var(--type-body-sm);
-}
-
 .starter-greeting-block h1 {
   margin: 0;
   color: var(--color-ink);
   font-family: var(--font-display);
-  font-size: clamp(26px, 3.2vw, 34px);
+  font-size: clamp(24px, 2.8vw, 30px);
   font-weight: 500;
   letter-spacing: -.02em;
-  line-height: 1.3;
+  line-height: 1.25;
   text-wrap: balance;
 }