refactor(css): 重构样式文件并优化主题样式

- 新增 theme.scss 文件，定义主题颜色- 新增 public.scss 文件，统一公共样式
- 修改 main.scss 文件，引入新的样式文件
-优化容器背景色渐变效果- 调整导航栏样式，增加面包屑导航
-统一问题类型样式，优化选择题样式

components.d.ts

@@ -17,14 +17,18 @@ declare module 'vue' {
     VanCellGroup: typeof import('vant/es')['CellGroup']
     VanCheckbox: typeof import('vant/es')['Checkbox']
     VanCheckboxGroup: typeof import('vant/es')['CheckboxGroup']
+    VanCol: typeof import('vant/es')['Col']
     VanDivider: typeof import('vant/es')['Divider']
     VanField: typeof import('vant/es')['Field']
+    VanGrid: typeof import('vant/es')['Grid']
+    VanGridItem: typeof import('vant/es')['GridItem']
     VanIcon: typeof import('vant/es')['Icon']
     VanNavBar: typeof import('vant/es')['NavBar']
     VanPicker: typeof import('vant/es')['Picker']
     VanPopup: typeof import('vant/es')['Popup']
     VanRadio: typeof import('vant/es')['Radio']
     VanRadioGroup: typeof import('vant/es')['RadioGroup']
+    VanRow: typeof import('vant/es')['Row']
     VanSearch: typeof import('vant/es')['Search']
     VanStepper: typeof import('vant/es')['Stepper']
     VanSwitch: typeof import('vant/es')['Switch']