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/repository/user_repo.go

@@ -2,6 +2,7 @@ package repository
 
 import (
 	"carrot_bbs/internal/model"
+	"strings"
 
 	"gorm.io/gorm"
 	"gorm.io/gorm/clause"
@@ -341,8 +342,12 @@ func (r *UserRepository) Search(keyword string, page, pageSize int) ([]*model.Us
 				keyword,
 			)
 		} else {
-			searchPattern := "%" + keyword + "%"
-			query = query.Where("username LIKE ? OR nickname LIKE ? OR bio LIKE ?", searchPattern, searchPattern, searchPattern)
+			// SQLite: 使用 LOWER() 实现大小写不敏感的模糊搜索
+			lowerSearchPattern := "%" + strings.ToLower(keyword) + "%"
+			query = query.Where(
+				"LOWER(username) LIKE ? OR LOWER(nickname) LIKE ? OR LOWER(bio) LIKE ?",
+				lowerSearchPattern, lowerSearchPattern, lowerSearchPattern,
+			)
 		}
 	}