feat(group): refactor group invite workflow for improved user experience

- Simplify InviteMembers to always send invite card directly to invitee
- Modify RespondInvite to check group join type (anyone vs need approval) before adding member
- Update SetGroupAddRequest to properly handle invite flow with invitee agreement and admin approval stages
- Add case-insensitive search using LOWER() in user repository
- Add random offset (0-50) for first page of recommended posts
- Update dependencies: remove gorilla/websocket, promote gomail to direct dependency

internal/handler/message_handler.go

@@ -423,7 +423,7 @@ func (h *MessageHandler) HandleSendMessage(c *gin.Context) {
 		response.BadRequest(c, "detail_type is required")
 		return
 	}
-	if params.Segments == nil || len(params.Segments) == 0 {
+	if len(params.Segments) == 0 {
 		response.BadRequest(c, "segments is required")
 		return
 	}