feat(chat): port chat widget components from Dart to React

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

@@ -0,0 +1,32 @@
+"use client";
+/**
+ * ChatSendButton 发送按钮
+ *
+ * 原始 Dart: lib/ui/chat/widgets/chat_send_button.dart（37 行）
+ *
+ * 视觉：
+ *   - 默认：粉色背景 + 渐变边缘
+ *   - 聚焦 / 激活：完整 primaryGradient（accent → 透明）
+ */
+import styles from "./chat-send-button.module.css";
+
+export interface ChatSendButtonProps {
+  disabled: boolean;
+  onClick: () => void;
+}
+
+export function ChatSendButton({ disabled, onClick }: ChatSendButtonProps) {
+  return (
+    <button
+      type="button"
+      className={`${styles.button} ${disabled ? "" : styles.buttonActive}`}
+      disabled={disabled}
+      onClick={onClick}
+      aria-label="Send message"
+    >
+      <span className={styles.icon} aria-hidden="true">
+        ↑
+      </span>
+    </button>
+  );
+}