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/DESIGN.md

@@ -471,10 +471,16 @@ The empty state is the composer. It used to be a bordered hero card at
 sitting on top of the input, so the primary action was the last thing on the
 page. Three parts now, in reading order:
 
-- **Greeting** (`.starter-greeting-block`): a tertiary-ink salutation line and
-  one question at `clamp(26px, 3.2vw, 34px)`, weight 500, centred. It lives in
-  the conversation area with `align-content: end`, so it sits directly above the
-  composer rather than floating in the middle of the row.
+- **Greeting** (`.starter-greeting-block`): one line, no subtitle — a single h1
+  at `clamp(24px, 2.8vw, 30px)`, weight 500, `line-height: 1.25`, centred. The
+  salutation line above the question is gone; so is the question, which is the
+  composer placeholder's job now («想聊什么都可以»). The line is drawn per visit
+  from three pools — daypart (morning / noon / afternoon / evening / late-night),
+  weekday (Mon, Fri, Sat, Sun only), and returning (accounts that already have a
+  session) — concatenated in that order and indexed by one random seed, with the
+  `{名字}` entries dropped when no name is set. It lives in the conversation area
+  with `align-content: end`, so it sits directly above the composer rather than
+  floating in the middle of the row.
 - **Composer**: unchanged, and *in the same DOM position it occupies during a
   live session*. Nothing remounts when the first message lands, so an in-flight
   draft, focus, or queued card survives the transition. Its width is