Implement friend APIs and target profile support

- Add friend search/list/request/requestCancel/response/expel handlers and schemas
- Store friend relationships in user_friend and backfill default friend links at startup
- Auto-add the default user as a friend for newly created accounts
- Support target user_id in /api profile requests and return target profile data
- Make profile accessory_info optional to avoid zero-value accessory payloads
- Update login topInfo friend counters from friend relationship state

Signed-off-by: Sean Du <do4suki@gmail.com>

internal/handler/ghome/account/login.go

@@ -362,6 +362,16 @@ func addUser(dbSession *xorm.Session, phone, password string, isDefault bool) (g
 			}
 		}
 
+		if !isDefault {
+			err = usermodel.EnsureDefaultFriendship(dbSession, userID)
+			if err != nil {
+				if localSession {
+					dbSession.Rollback()
+				}
+				return loginData, loginCode, loginMsg, created, err
+			}
+		}
+
 		loginData.Autokey = autoKey
 		loginData.HasRealInfo = 1
 		loginData.Message = "ok"