reservation: combine initialize-seat and fund into pay command

[martinsander00]

Summary

• Combine the two-step reservation initialize-seat + reservation fund workflow into a single reservation pay command
• The new command checks for existing seat/escrow accounts on-chain and only initializes what's missing before funding
• Removes the initialize-seat subcommand entirely

Lines of Code

Section	Added	Removed
reservation CLI	+77	-117

Testing Verification

• Tested via on-chain integration tests in the shreds repo