Persist the printer token on connect (survive unclean restarts)

[VGottselig]

Problem

The printer token is only written to the known-hosts file by the shutdown defer in main(), which runs only on a graceful exit. On an unclean exit — SIGKILL, container restart, OOM, power loss — a freshly authorized token is lost, so the next start needs another touchscreen authorization.

There is also a second gap: the IP-only fallback printer (-host <ip> without discovery) is created without an ID:

printer = &Printer{IP: Host}

but LocalStorage.Add skips ID-less printers:

func (ls *LocalStorage) Add(printers ...*Printer) {
    for _, p := range printers {
        if p.ID == "" {
            continue   // <- fallback printer never stored
        }
        ...

so that printer's token is never persisted, even on a clean exit.

Fix

• Add an OnPrinterUpdate hook, invoked right after a successful connect, that main() wires to ls.Add(printer) + ls.Save() — persisting the (refreshed) token immediately instead of only at shutdown.
• Give the IP-only fallback printer an ID (its host) so ls.Add actually persists it.

No change to the normal shutdown path; this just makes persistence happen eagerly and closes the ID-less-fallback gap.

Testing

• go build ./... clean.
• Token now survives docker kill / container restarts and unclean power cycles — no re-authorization on the touchscreen on the next upload.