The first time you open the web app, the browser warns about the certificate. That's expected: the server made the certificate itself, because nothing on a home network can get a public one. Accept it for this address.
When it comes back
The certificate names the machine's addresses on your home network and its local name. If the machine's address changes, the browser may warn again. A fixed address, with a DHCP reservation in your router, avoids it.
When the machine's addresses change, the server issues a new certificate on the same key, so paired phones keep working without pairing again.
Phones
Phones don't see the warning: Piktures pins the server's key at pairing, and checks it on every connection. See Pair a phone.