Offload DH key exchange crypto to background thread

[shiel-pty]

Summary

• MTP::CreateModExp (a while(true) BigNum modular exponentiation loop) and MTP::CreateAuthKey were running synchronously on the main thread during call setup, freezing the UI for tens to hundreds of milliseconds
• Moved all three call sites (start(), confirmAcceptedCall(), startConfirmedCall()) to crl::async with results posted back via crl::on_main
• generateModExpFirst was removed as a separate method; its logic is now inlined in the start() async callback
• startConfirmedCall signature changed to take MTPPhoneCall by value so it can be safely moved into the async lambda

Test plan

• Outgoing call: initiate a call and verify the UI remains responsive while the call connects
• Incoming call: answer an incoming call and verify no UI freeze during key exchange
• Incoming call answered before DH config arrives: verify _answerAfterDhConfigReceived path still works correctly
• Call hangup during key exchange: verify no crash when the call is ended while async is in flight

[CLAassistant]

All committers have signed the CLA.