feat: enable paykit by default

[ovitrif]

Problem or use case

Paykit ships in 2.5.0, but only behind the "Enable Paykit UI" switch in Dev Settings, which is off by default. In 2.6.0, Paykit goes to production for every user.

Proposed solution

• Paykit is on by default for new installs and for wallets updating from 2.5.0.
• The Dev Settings switch stays, so testers can turn Paykit off and back on.
• The PAYKIT_UI_DISABLED build flag still hides Paykit in a build where it is set, so a hotfix build can turn Paykit off in production.

Acceptance

• A fresh install shows Paykit without any change in Dev Settings.
• A wallet updated from 2.5.0 shows Paykit after the update, unless someone turned it off in Dev Settings before.
• Turning the Dev Settings switch off hides Paykit and removes its published endpoints; turning it back on restores Paykit.

Alternatives considered

A visible opt-in switch in Settings, labelled Beta. Not chosen: Paykit goes to production on by default in 2.6.0.

Additional context

Pairs with synonymdev/bitkit-ios#802.