It's tough when you're bootstrapping a web app and
still have a day job.
I think you should tell your day-job about your potential business before you start. Get them to sign something that says
1) You agree not to use equipment/premises belonging to $dayjob to build $idea
2) You will never work on $idea during any hours you're getting paid by $dayjob
3) $idea does not directly compete with $dayjob
4) They recognize that they have no claim to $idea or anything else you build to support it
If $dayjob is a large enough to have a dedicated HR dept they probably have a specific form to fill out.