cli: block shreds pay when device has no available seats

[martinsander00]

Summary

• Adds a client-side guard to shreds pay that checks the device's granted_seat_count against total_available_seats from the on-chain DeviceHistory account
• Prevents users from submitting a transaction that would fail on-chain with an opaque error when the device is full
• Skips the check when the seat is already active (re-fund scenario, no new allocation needed)

Lines of Code

Section	Added	Removed
cli (pay.rs)	+10	-0
changelog	+1	-0

Testing Verification

• Verified the guard uses the same capacity check as the on-chain program (active_granted_seats >= active_total_available_seats)
• The on-chain program enforces this constraint as a backstop; this guard provides a clear client-side error message