fix(chat): make send button circular

src/app/chat/components/chat-send-button.module.css

@@ -2,10 +2,11 @@
 
 .button {
   flex: 0 0 auto;
-  width: 48px;
+  width: 40px;
   height: 40px;
+  aspect-ratio: 1 / 1;
   border: 0;
-  border-radius: 9999px;
+  border-radius: 50%;
   background: var(--color-button-gradient-end, #fc69df);
   color: #fff;
   cursor: pointer;

src/app/chat/components/chat-send-button.tsx

@@ -8,9 +8,9 @@
  *   - 默认：粉色背景 + 渐变边缘
  *   - 聚焦 / 激活：完整 primaryGradient（accent → 透明）
  *
- * 图标：lucide-react <Send />（tree-shakable，currentColor 继承父 color）
+ * 图标：lucide-react <ArrowUp />（tree-shakable，currentColor 继承父 color）
  */
-import { ArrowUp, Send } from "lucide-react";
+import { ArrowUp } from "lucide-react";
 
 import styles from "./chat-send-button.module.css";