feat(posts): add @mention highlighting and vote segment display

- Parse post content to highlight @mentions with blue styling
- Render vote segments from post data with formatted options display
- Add MessageSegment interface and segments/is_vote fields to Post type
- Update API base URL and branding from Carrot BBS to WithYou

.env.development

@@ -1,2 +1,2 @@
 # 开发环境 API 地址
-VITE_API_BASE_URL=https://bbs.littlelan.cn/api/v1
+VITE_API_BASE_URL=https://withyou.littlelan.cn/api/v1