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

frontend/src/app/(app)/page.tsx

@@ -12,6 +12,7 @@ import type { BirthTimeAssessmentPhase } from "@/components/birth-time-assessmen
 import { AppLoadingIndicator } from "@/components/app-loading-indicator";
 import { HomeBootstrapError } from "@/components/home-bootstrap-error";
 import { HomeOnboardingShell } from "@/components/home-onboarding-shell";
+import { BirthSkyReveal } from "@/components/birth-sky/birth-sky-reveal";
 import {
   entrySummaryFromResponse,
   rectificationEntryLabels,
@@ -66,6 +67,7 @@ import type { ConsultationBirthTimeMode } from "@/lib/consultation-birth-time-mo
 import { useBirthTimeGuidedJourney } from "@/hooks/use-birth-time-guided-journey";
 import { useConsultationRun } from "@/hooks/use-consultation-run";
 import { useConversationScrollAnchor } from "@/hooks/use-conversation-scroll-anchor";
+import { useBirthSkyReveal } from "@/hooks/use-birth-sky-reveal";
 import { useProfileOnboarding } from "@/hooks/use-profile-onboarding";
 import {
   createRectificationShellSetters,
@@ -297,6 +299,7 @@ export default function Home() {
   const uiPreview = useRef(false);
   const uiPreviewMode = useRef<string | null>(null);
   const birthTimeRevisionPendingRef = useRef(false);
+  const birthSkyReveal = useBirthSkyReveal();
   const {
     profile,
     setProfile,
@@ -330,6 +333,7 @@ export default function Home() {
     setRectificationError,
     setSigningOut, setStartGreeting, signingOut, uiPreview,
     initialProfile: () => warmStart()?.profile ?? null,
+    onOwnBirthPlaceSaved: birthSkyReveal.begin,
   });
 
   const birthTimeGuided = useBirthTimeGuidedJourney({
@@ -1349,6 +1353,7 @@ export default function Home() {
           ? <div className="starter-balance" aria-hidden="true" />
           : null}
 
+      <BirthSkyReveal cover={birthSkyReveal.cover} onClose={birthSkyReveal.close} />
       <AccountDialogOverlay
         open={activeAccountDialog !== null}
         dialog={activeAccountDialog}

frontend/src/components/birth-sky/birth-sky-reveal.tsx

@@ -0,0 +1,15 @@
+"use client";
+
+import dynamic from "next/dynamic";
+
+import type { BirthSkyCover } from "@/lib/birth-sky/cover";
+
+const BirthSkyDialog = dynamic(
+  () => import("./birth-sky-dialog").then((module) => module.BirthSkyDialog),
+  { ssr: false },
+);
+
+/** First-chart reveal: the chart page's dialog, faded in once, with 「继续」. */
+export function BirthSkyReveal({ cover, onClose }: { cover: BirthSkyCover | null; onClose: () => void }) {
+  return cover ? <BirthSkyDialog cover={cover} onClose={onClose} reveal /> : null;
+}

frontend/src/components/birth-sky/birth-sky.css

@@ -15,9 +15,6 @@
   color: var(--color-ink);
   overflow: hidden;
 }
-@supports (height: 1dvh) {
-  .birth-sky-dialog { height: 100dvh; }
-}
 .birth-sky-dialog::backdrop { background: var(--color-scrim); }
 .birth-sky-panel { display: flex; flex-direction: column; height: 100%; }
 .birth-sky-header {
@@ -42,11 +39,16 @@
   width: auto;
   height: auto;
   max-width: 100%;
-  max-height: 100%;
+  /* Header + footer ≈ 170px; a percentage would not resolve inside the flex/grid cell everywhere. */
+  max-height: calc(100vh - 170px);
   aspect-ratio: 3 / 4;
   border-radius: var(--radius-md);
   -webkit-touch-callout: default;
 }
+@supports (height: 1dvh) {
+  .birth-sky-dialog { height: 100dvh; }
+  .birth-sky-image { max-height: calc(100dvh - 170px); }
+}
 .birth-sky-footer {
   display: flex;
   flex-wrap: wrap;

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

@@ -0,0 +1,37 @@
+"use client";
+
+import { useCallback, useEffect, useRef, useState } from "react";
+
+import { loadBirthSkyCover, releaseBirthSkyCover, type BirthSkyCover } from "@/lib/birth-sky/cover";
+import { startBirthSkyReveal } from "@/lib/birth-sky/reveal";
+
+/**
+ * The one-time 「那一刻的天空」 reveal after the reader's own first chart
+ * (TASK-birth-sky-cover-20260928 T6). Owns its state so Home only wires it:
+ * `begin` goes to the onboarding hook, `cover` / `close` to the dialog.
+ * Saving one's own birth place happens once per account, so no "seen" flag.
+ */
+export function useBirthSkyReveal() {
+  const [cover, setCover] = useState<BirthSkyCover | null>(null);
+  const cancel = useRef<(() => void) | null>(null);
+
+  const begin = useCallback(() => {
+    cancel.current?.();
+    cancel.current = startBirthSkyReveal({
+      load: (signal) => loadBirthSkyCover("self:reveal", "self", signal),
+      onReady: (next) => setCover(next),
+      onDiscard: releaseBirthSkyCover,
+    });
+  }, []);
+
+  const close = useCallback(() => {
+    setCover((current) => {
+      releaseBirthSkyCover(current);
+      return null;
+    });
+  }, []);
+
+  useEffect(() => () => { cancel.current?.(); }, []);
+
+  return { cover, begin, close };
+}

frontend/src/hooks/use-profile-onboarding.ts

@@ -83,6 +83,8 @@ export type ProfileOnboardingParams = {
   uiPreview: MutableRefObject<boolean>;
   /** Warm return to `/` (BUG-1040): the profile Home starts with instead of empty. */
   initialProfile?: () => Profile | null;
+  /** The reader's own birth place was saved in onboarding (first chart): 「那一刻的天空」 reveal. */
+  onOwnBirthPlaceSaved?: () => void;
 };
 
 export function useProfileOnboarding(params: ProfileOnboardingParams) {
@@ -111,6 +113,7 @@ export function useProfileOnboarding(params: ProfileOnboardingParams) {
     signingOut,
     uiPreview,
     initialProfile,
+    onOwnBirthPlaceSaved,
   } = params;
 
   // Owned here since 2026-09-16 (state lowering batch 2). Home reads them back
@@ -379,6 +382,8 @@ export function useProfileOnboarding(params: ProfileOnboardingParams) {
       setStartGreeting(createStartGreeting(savedProfile.name));
       setPresetMessageLength(0);
       setOnboardingJustCompleted(false);
+      // Fire and forget: the flow above has already moved on and never waits for it.
+      onOwnBirthPlaceSaved?.();
     } catch (caught) {
       setAccountError(friendlyError(caught instanceof Error ? caught.message : "出生地点保存失败"));
     } finally {

frontend/src/lib/birth-sky/reveal.ts

@@ -0,0 +1,54 @@
+import type { BirthSkyCover } from "./cover.ts";
+
+/**
+ * First-chart reveal timing (TASK-birth-sky-cover-20260928 T6).
+ *
+ * After the reader's own birth place is saved, the cover gets one chance: if
+ * it is fetched and painted within `timeoutMs` it is shown, otherwise it is
+ * dropped silently — no error, no retry, and the onboarding flow never waits.
+ */
+export const BIRTH_SKY_REVEAL_TIMEOUT_MS = 3_000;
+
+export type BirthSkyRevealInput = Readonly<{
+  load: (signal: AbortSignal) => Promise<BirthSkyCover | null>;
+  onReady: (cover: BirthSkyCover) => void;
+  /** A cover that arrived too late; the caller releases its object URL. */
+  onDiscard?: (cover: BirthSkyCover) => void;
+  timeoutMs?: number;
+  setTimer?: (callback: () => void, ms: number) => unknown;
+  clearTimer?: (handle: unknown) => void;
+}>;
+
+/** Starts one reveal attempt; the returned function cancels it. */
+export function startBirthSkyReveal(input: BirthSkyRevealInput): () => void {
+  const setTimer = input.setTimer ?? ((callback: () => void, ms: number) => setTimeout(callback, ms));
+  const clearTimer = input.clearTimer ?? ((handle: unknown) => clearTimeout(handle as ReturnType<typeof setTimeout>));
+  const controller = new AbortController();
+  let settled = false;
+  const timer = setTimer(() => {
+    if (settled) return;
+    settled = true;
+    controller.abort();
+  }, input.timeoutMs ?? BIRTH_SKY_REVEAL_TIMEOUT_MS);
+  void input.load(controller.signal).then(
+    (cover) => {
+      if (settled || !cover) {
+        if (cover) input.onDiscard?.(cover);
+        return;
+      }
+      settled = true;
+      clearTimer(timer);
+      input.onReady(cover);
+    },
+    () => {
+      settled = true;
+      clearTimer(timer);
+    },
+  );
+  return () => {
+    if (settled) return;
+    settled = true;
+    clearTimer(timer);
+    controller.abort();
+  };
+}