fix: check payment proof size before paying #1842

Twin: synonymdev/bitkit-ios#1271

What

Bitkit pins Paykit 0.1.1-rc2. Paykit 0.1.1-rc6 lets Bitkit measure a Payment Proof with the SDK before paying, and the later Paykit releases for 3.2.0 build on it.

Expected

Bitkit uses Paykit 0.1.1-rc6, and:

• refuses a Lightning, on-chain or USDT Payment Request payment whose Payment Proof could never reach the payee, before anything is paid
• explains a Payment Request note that is too long to send
• explains a refused signup code or a network error during Pubky authorization
• doesn't count a USDT transfer made well before a Payment Request as paying it

See the PR that implements it: #1803