feat: add ChatButton floating action button

src/components/ChatButton/index.jsx

@@ -0,0 +1,23 @@
+import React from 'react';
+import styles from './styles.module.css';
+
+export default function ChatButton({ isOpen, onClick }) {
+  return (
+    <button
+      className={`${styles.btn} ${isOpen ? styles.btnOpen : ''}`}
+      onClick={onClick}
+      aria-label={isOpen ? '关闭 AI 助手' : '打开 AI 助手'}
+    >
+      {isOpen ? (
+        <svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+          <line x1="18" y1="6" x2="6" y2="18" />
+          <line x1="6" y1="6" x2="18" y2="18" />
+        </svg>
+      ) : (
+        <svg width="24" height="24" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+          <path d="M21 15a2 2 0 0 1-2 2H7l-4 4V5a2 2 0 0 1 2-2h14a2 2 0 0 1 2 2z" />
+        </svg>
+      )}
+    </button>
+  );
+}

src/components/ChatButton/styles.module.css

@@ -0,0 +1,50 @@
+.btn {
+  position: fixed;
+  bottom: 2rem;
+  right: 2rem;
+  width: 56px;
+  height: 56px;
+  border-radius: 50%;
+  border: none;
+  background: #466aff;
+  color: #fff;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  box-shadow: 0 4px 16px rgba(70, 106, 255, 0.4);
+  z-index: 1000;
+  transition: transform 0.2s ease, box-shadow 0.2s ease, background 0.2s ease;
+}
+
+.btn:hover {
+  transform: scale(1.08);
+  box-shadow: 0 6px 24px rgba(70, 106, 255, 0.5);
+}
+
+.btnOpen {
+  background: #666;
+  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.2);
+}
+
+.btnOpen:hover {
+  background: #555;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.3);
+}
+
+[data-theme='dark'] .btn {
+  background: #466aff;
+}
+
+[data-theme='dark'] .btnOpen {
+  background: #555;
+}
+
+@media (max-width: 768px) {
+  .btn {
+    bottom: 1.2rem;
+    right: 1.2rem;
+    width: 48px;
+    height: 48px;
+  }
+}