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/chat/components/pwa-install-dialog.tsx

@@ -7,6 +7,10 @@
  * 用途：引导用户将 Web App 安装到桌面
  * 触发：由 PwaInstallOverlay 在延迟 3.5s 后自动显示
  */
+import Image from "next/image";
+
+import { AppConstants } from "@/core/constants/app_constants";
+
 import styles from "./pwa-install-dialog.module.css";
 
 export interface PwaInstallDialogProps {
@@ -35,15 +39,19 @@ export function PwaInstallDialog({ onClose, onInstall }: PwaInstallDialogProps)
       aria-labelledby="pwa-dialog-title"
     >
       <div className={styles.dialog}>
-        <div className={styles.icon} aria-hidden="true">
-          📱
-        </div>
+        <Image
+          className={styles.icon}
+          src="/images/icons/icon-add-to-home.png"
+          alt=""
+          width={86}
+          height={86}
+          priority
+        />
         <h2 id="pwa-dialog-title" className={styles.title}>
-          Install cozsweet
+          Install {AppConstants.appTitle}
         </h2>
         <p className={styles.content}>
-          Add to your home screen for the best experience. Faster loading,
-          full-screen mode, and offline support.
+          {AppConstants.appTitle} can be installed on your device for a faster, full-screen experience.
         </p>
         <div className={styles.actions}>
           <button
@@ -51,14 +59,14 @@ export function PwaInstallDialog({ onClose, onInstall }: PwaInstallDialogProps)
             className={`${styles.btn} ${styles.btnSecondary}`}
             onClick={onClose}
           >
-            Not now
+            Cancel
           </button>
           <button
             type="button"
             className={`${styles.btn} ${styles.btnPrimary}`}
             onClick={handleInstall}
           >
-            Install
+            OK
           </button>
         </div>
       </div>