feat(onboarding): no hint toast after the first-chart star convergence

Product 2026-09-29: drop 「这片天空在星盘页可以保存。」. The chart page entry is now replay + share.
Verified: tsc 0, lint 0 errors, npm test 4335/4280/24 (same 24 Docker/DB/deploy names),
quick gate (gate params) passed, / Static, first-screen JS gzip 651,512 -> 651,143.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4f2nya58RoRu4yEmJgRGE

frontend/src/hooks/use-birth-sky-reveal.ts

@@ -1,25 +1,21 @@
 "use client";
 
 import { useCallback, useEffect, useRef, useState } from "react";
-import { toast } from "sonner";
 
 import { loadBirthSkyRevealSky, startBirthSkyReveal, type BirthSkyRevealSky } from "@/lib/birth-sky/reveal";
 
-/** Shown once on the home page after the convergence ends (VOICE.md). */
-export const BIRTH_SKY_REVEAL_HINT = "这片天空在星盘页可以保存。";
-
 /**
  * The one-time 「那一刻的天空」 convergence after the reader's own first chart
  * (TASK-birth-sky-cover-20260928 T6 → TASK-birth-sky-followup-20260928 F4).
  * Owns its state so Home only wires it: `begin` goes to the onboarding hook,
  * `cover` / `close` to the overlay. `cover` is the sky plus the serif verdict,
  * ready before the first frame. Saving one's own birth place happens once per
- * account, so no "seen" flag.
+ * account, so no "seen" flag. Nothing follows the overlay: the hint toast was
+ * removed by the product on 2026-09-29.
  */
 export function useBirthSkyReveal() {
   const [cover, setCover] = useState<BirthSkyRevealSky | null>(null);
   const cancel = useRef<(() => void) | null>(null);
-  const playing = useRef(false);
 
   const begin = useCallback(() => {
     cancel.current?.();
@@ -32,19 +28,11 @@ export function useBirthSkyReveal() {
         ]);
         return sky;
       },
-      onReady: (next) => {
-        playing.current = true;
-        setCover(next);
-      },
+      onReady: (next) => setCover(next),
     });
   }, []);
 
-  const close = useCallback(() => {
-    if (!playing.current) return;
-    playing.current = false;
-    setCover(null);
-    toast(BIRTH_SKY_REVEAL_HINT);
-  }, []);
+  const close = useCallback(() => setCover(null), []);
 
   useEffect(() => () => { cancel.current?.(); }, []);