fix: potential panics in login and return proper http 405 (#8065)

# Which Problems Are Solved

We identified some parts in the code, which could panic with a nil
pointer when accessed without auth request.
Additionally, if a GRPC method was called with an unmapped HTTP method,
e.g. POST instead of GET a 501 instead of a 405 was returned.

# How the Problems Are Solved

- Additional checks for existing authRequest
- custom http status code mapper for gateway

# Additional Changes

None.

# Additional Context

- noted internally in OPS

internal/api/ui/login/mail_verify_handler.go

@@ -58,16 +58,17 @@ func (l *Login) handleMailVerificationCheck(w http.ResponseWriter, r *http.Reque
 		l.checkMailCode(w, r, authReq, data.UserID, data.Code)
 		return
 	}
-	userOrg := ""
+	var userOrg, authReqID string
 	if authReq != nil {
 		userOrg = authReq.UserOrgID
+		authReqID = authReq.ID
 	}
 	emailCodeGenerator, err := l.query.InitEncryptionGenerator(r.Context(), domain.SecretGeneratorTypeVerifyEmailCode, l.userCodeAlg)
 	if err != nil {
 		l.checkMailCode(w, r, authReq, data.UserID, data.Code)
 		return
 	}
-	_, err = l.command.CreateHumanEmailVerificationCode(setContext(r.Context(), userOrg), data.UserID, userOrg, emailCodeGenerator, authReq.ID)
+	_, err = l.command.CreateHumanEmailVerificationCode(setContext(r.Context(), userOrg), data.UserID, userOrg, emailCodeGenerator, authReqID)
 	l.renderMailVerification(w, r, authReq, data.UserID, err)
 }