feat: 侧边栏分组图标与导航样式优化

- 分组支持 icon 字段，可按名称自动匹配或手动选择
- 左侧导航与总览卡片显示彩色 SVG 图标
- 优化侧栏链接圆角与选中态样式

Co-authored-by: Cursor <cursoragent@cursor.com>

forms.py

@@ -11,6 +11,22 @@ class LoginForm(FlaskForm):
 
 class GroupForm(FlaskForm):
     name = StringField("分组名称", validators=[DataRequired(message="请输入分组名称")])
+    icon = SelectField(
+        "图标",
+        choices=[
+            ("", "自动（按分组名匹配）"),
+            ("order", "下单 / 订单"),
+            ("trade", "交易 / 行情"),
+            ("review", "复盘 / 历史"),
+            ("api", "API / 接口"),
+            ("gate", "Gate / 扫单"),
+            ("chart", "图表 / K线"),
+            ("server", "服务器 / 面板"),
+            ("folder", "文件夹（通用）"),
+        ],
+        default="",
+        validators=[Optional()],
+    )
     sort_order = IntegerField(
         "排序（越小越靠前）",
         validators=[Optional(), NumberRange(min=-10**6, max=10**6)],