fix UseWithCtx method to check for nil context

- Add nil check for context parameter.
- UseWithCtx method return explicit type rather than interface.

Replace base64Captcha.Store return type with *RedisStore for better type safety.

server/utils/captcha/redis.go

@@ -5,7 +5,6 @@ import (
 	"time"
 
 	"github.com/flipped-aurora/gin-vue-admin/server/global"
-	"github.com/mojocn/base64Captcha"
 	"go.uber.org/zap"
 )
 
@@ -23,8 +22,10 @@ type RedisStore struct {
 	Context    context.Context
 }
 
-func (rs *RedisStore) UseWithCtx(ctx context.Context) base64Captcha.Store {
-	rs.Context = ctx
+func (rs *RedisStore) UseWithCtx(ctx context.Context) *RedisStore {
+	if ctx == nil {
+		rs.Context = ctx
+	}
 	return rs
 }