refactor: 移除工具栏 Gitee 按钮，项目地址移至关于弹框

- 工具栏移除 Gitee 图标按钮
- 关于弹框添加项目地址链接（带 Gitee 图标）
- 链接点击跳转到 gitee.com/thzxx/MarkLite

src/renderer/styles/global.css

@@ -1161,6 +1161,26 @@ html, body {
   margin-bottom: 20px;
 }
 
+.about-link {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  color: var(--primary);
+  text-decoration: none;
+  font-size: 13px;
+  margin-bottom: 8px;
+  padding: 6px 12px;
+  border-radius: 6px;
+  background: var(--primary-light);
+  transition: all 0.15s ease;
+  cursor: pointer;
+}
+
+.about-link:hover {
+  background: var(--primary);
+  color: white;
+}
+
 .about-footer p {
   color: var(--text-tertiary);
   font-size: 12px;