feat(config): ThemeResp 增加品牌字段 + 公开品牌信息端点

- ThemeResp 新增 brand_name/brand_slogan/brand_features/brand_copyright 字段
- default_theme 提供品牌默认值
- 新增 PublicBrandResp 和 GET /api/v1/public/brand 公开端点（无需认证）
- ConfigModule 增加 public_routes 方法
- 更新测试覆盖品牌字段

crates/erp-config/src/dto.rs

@@ -224,6 +224,23 @@ pub struct ThemeResp {
     pub logo_url: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
     pub sidebar_style: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub brand_name: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub brand_slogan: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub brand_features: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub brand_copyright: Option<String>,
+}
+
+/// 品牌信息公开响应（不含内部配置）
+#[derive(Debug, Serialize, Deserialize, ToSchema, Clone)]
+pub struct PublicBrandResp {
+    pub brand_name: String,
+    pub brand_slogan: String,
+    pub brand_features: String,
+    pub brand_copyright: String,
 }
 
 // --- Language DTOs (stored via settings) ---