feat(onboarding): one-time 「那一刻的天空」 reveal after the first own chart (T6)

After saveOnboardingPlace succeeds (and the next step is already set), the
new useBirthSkyReveal hook fetches /api/birth-sky?subject=self and paints
the cover; if that finishes within 3 s the chart page's dialog fades in
(600 ms, none under reduced motion) with 「继续」. Timeout or failure is
silent, no retry; closing touches only the reveal's own state. Home() gains
no useState/useRef; page.tsx only wires. Family profiles and profile edits
never trigger it.

Also caps the dialog image by viewport height (the percentage max-height did
not resolve inside the flex/grid cell), and completes PROGRESS, DESIGN §17,
CHANGELOG and the real-device checklist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0199rbQDTsUbCVw84wc8BTFe

docs/testing/birth-sky-cover-checklist.md

@@ -33,3 +33,13 @@
 | --- | --- | --- | --- | --- |
 | 11 | 桌面 Chrome | 打开「我的报告」任一份 | 没有天空封面，没有占位 | |
 | 12 | 桌面 Chrome | 该报告「打印 → 存为 PDF」 | 第一页与上线前相同 | |
+
+## 首次建盘揭幕
+
+| # | 设备 / 浏览器 | 操作 | 期望 | 结果 |
+| --- | --- | --- | --- | --- |
+| 13 | iPhone Safari | 新账号走完首页引导，最后一步保存出生地 | 3 秒内在当前页面上淡入同一张图（约 0.6 秒），多一个「继续」 | |
+| 14 | 同上 | 点「继续」/ Esc / 点遮罩 | 关掉，下面就是原本的下一步（问候或校正引导），没有被换掉 | |
+| 15 | 桌面 Chrome | 新账号，保存出生地前在 DevTools → Network → Request blocking 加一条 `*api/birth-sky*`（或把 `/api/birth-sky` 限速到 3 秒以上） | 保存成功后不出现弹层、不报错，引导照常往下走 | |
+| 16 | 任一 | 系统开启「减少动态效果」后重复 13 | 弹层直接出现，没有淡入 | |
+| 17 | 任一 | 在星盘档案里新建一个家人、或在设置里改自己的出生资料 | 都不弹「那一刻的天空」 | |