feat(web,engine): portrait-orientation scene images for mobile full-bleed

Thread orientation (portrait|landscape) from client through API, engine,
and image gen. Portrait devices render 1024x1792 (9:16) full-bleed scenes;
desktop/landscape keeps 1792x1024 (16:9). Adds cover-aware click→image
coordinate mapping, session-locked orientation, a shared coerceOrientation
helper, and a choices overflow cap in portrait.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/layout.tsx

@@ -1,4 +1,4 @@
-import type { Metadata } from "next";
+import type { Metadata, Viewport } from "next";
 import { Cormorant_Garamond, Inter } from "next/font/google";
 import { Analytics } from "@/components/Analytics";
 import "./globals.css";
@@ -25,6 +25,15 @@ export const metadata: Metadata = {
   description: "InfiPlot 是一款用 AI 实时生成图片、语音与剧情分支的交互式剧情游戏 Demo。",
 };
 
+// viewportFit:cover lets the immersive /play portrait layout extend under the
+// iOS notch / home-indicator and exposes env(safe-area-inset-*) to the
+// floating controls. device-width + initialScale keep mobile rendering 1:1.
+export const viewport: Viewport = {
+  width: "device-width",
+  initialScale: 1,
+  viewportFit: "cover",
+};
+
 export default function RootLayout({
   children,
 }: {