style(pwa-install-dialog): redesign dialog to match Dart spec

Update the PWA install dialog styling and content to align with the
original Dart design, including a white background with large 40px
radius, 86x86 PNG icon, gradient primary button, and dynamic app title
sourced from AppConstants.

src/app/layout.tsx

@@ -1,4 +1,4 @@
-import type { Metadata } from "next";
+import type { Metadata, Viewport } from "next";
 import { SessionProvider } from "@/providers/session-provider";
 import { geistSans, geistMono, athelas } from "@/lib/fonts";
 import "./globals.css";
@@ -10,8 +10,6 @@ export const metadata: Metadata = {
   description: "cozsweet frontend (Next.js)",
   // PWA：链接到 public/manifest.json（由 @ducanh2912/next-pwa 体系使用）
   manifest: "/manifest.json",
-  // 浏览器顶栏 / Android PWA 顶栏颜色
-  themeColor: "#f84d96",
   // iOS Safari "添加到主屏" 元数据
   appleWebApp: {
     capable: true,
@@ -25,6 +23,16 @@ export const metadata: Metadata = {
   },
 };
 
+/**
+ * Next.js 14+ 推荐：viewport 相关元数据独立导出。
+ * 之前放在 `metadata.themeColor` 会在 Next.js 16 触发 deprecation 警告。
+ * 详见 https://nextjs.org/docs/app/api-reference/functions/generate-viewport
+ */
+export const viewport: Viewport = {
+  // 浏览器顶栏 / Android PWA 顶栏颜色
+  themeColor: "#f84d96",
+};
+
 export default function RootLayout({
   children,
 }: Readonly<{