开源准备：重写 README 展示 UI/UX，更新模块路径

- 新增界面截图与 CONTRIBUTING.md
- README 以三栏布局、暗色主题、移动端等视觉亮点为主
- go module 迁移至 git.iioio.com/freefire/jiang13-forum

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

middleware/auth.go

@@ -6,8 +6,8 @@ import (
 	"strings"
 
 	"github.com/gin-gonic/gin"
-	"github.com/jiang13/forum/model"
-	"github.com/jiang13/forum/service"
+	"git.iioio.com/freefire/jiang13-forum/model"
+	"git.iioio.com/freefire/jiang13-forum/service"
 )
 
 const (

middleware/presence.go

@@ -5,7 +5,7 @@ import (
 	"encoding/hex"
 
 	"github.com/gin-gonic/gin"
-	"github.com/jiang13/forum/service"
+	"git.iioio.com/freefire/jiang13-forum/service"
 )
 
 const VisitorCookieName = "j13_vid"