Add CLI flag to set DNS query payload size (DNS MTU)

[Nyxil]

Problem
The Android app supports a DNS query payload size cap (dnsPayloadSize), but the SlipNet CLI has no equivalent setting. This makes it impossible to tune DNS query size from the CLI for DNSTT/NoizDNS connections.

On some networks, large DNS queries are fragmented or blocked. The Android app can reduce DNS payload size to improve reliability, but CLI users cannot.

Current behavior
slipnet --help only shows --dns, --direct, --port, --host, --utls for connect. The only payload-size logic in the CLI is for scanner EDNS0 probing, not for the client path.

Expected behavior
Add a CLI flag such as --dns-payload <bytes> (or --payload / --mtu) to set the max DNS payload size for DNSTT/NoizDNS.

Example
slipnet --dns-payload 1232 slipnet://BASE64...

Implementation suggestion

• Add the flag in the CLI connect path.
• Plumb it into the DNSTT/NoizDNS client (noizdns/mobile) so it sets the max payload.
• Optional: allow this flag to override any value encoded in slipnet:// configs.