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">

frontend/src/components/pages/MobileCallPage.tsx

@@ -220,7 +220,7 @@ function MobileCallVisualWorkspace({
             ref={floatingVideoRef}
             type="button"
             style={{ left: position.x, top: position.y }}
-            className="absolute z-10 aspect-video w-[clamp(7.5rem,38vw,10rem)] touch-none overflow-hidden rounded-2xl border border-white/20 bg-black shadow-2xl ring-1 ring-black/20 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-white/50"
+            className="absolute z-10 aspect-[3/4] w-[clamp(7rem,34vw,9.25rem)] touch-none overflow-hidden rounded-2xl border border-white/20 bg-black shadow-2xl ring-1 ring-black/20 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-white/50 landscape:aspect-video landscape:w-[clamp(8rem,28vw,10rem)]"
             aria-label="拖动摄像头浮窗，点击切换到视频流"
             onPointerDown={(event) => {
               event.currentTarget.setPointerCapture(event.pointerId);