Update LoginPage placeholder text and refine MobileCallPage styling

- Change the placeholder text in LoginPage from "admin" to "请输入用户名" for better localization.
- Adjust the styling of the floating video panel in MobileCallPage to improve responsiveness and maintain aspect ratio across different screen sizes.

frontend/src/app/login/page.tsx

@@ -72,7 +72,7 @@ export default function LoginPage() {
                 autoComplete="username"
                 value={username}
                 onChange={(event) => setUsername(event.target.value)}
-                placeholder="admin"
+                placeholder="请输入用户名"
               />
             </label>
             <label className="block space-y-2">