fix(cli): don't crash when the public IP can't be resolved

[lazerg]

On a network where the public IP can't be resolved (blocked DNS, for example), slidev --remote=<password> dies before printing anything. publicIpv4() rejects there, and neither call site guards it: the dev server one takes down the process with DOMException [TimeoutError], and the c (qrcode) shortcut leaves an unhandled rejection.

The public IP only adds one extra "remote control" URL, so both calls now go through a small helper that catches the failure and warns, the same way the failed-to-open-browser path does. printInfo already treats the IP as optional and still prints the LAN URLs.

There's no test harness for cli.ts here, so I checked it by hand against the starter demo with public-ip stubbed to reject with the same TimeoutError. On main it crashes at startup; on this branch it warns, serves the deck, and still renders the QR code. With the stub removed the public IP shows up as before.

Fixes #2724

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commit: 8607338