Add a CSV export

[pm47]

Usage:

$ ./phoenix-cli exportcsv
payment history has been exported to /home/<user>/.phoenix/exports/export-1728035509.csv

The resulting CSV allows precise tracking of the balance and fee credit, and shows the split between mining and service fees:

• balance: sum of all amount_msat
• fee credit: sum of all fee_credit_msat

Columns:

date	type	amount_msat	fee_credit_msat	mining_fee_sat	service_fee_msat	payment_hash	tx_id

Fixes #36.

[t-bast]

Looks good to me, just naming nits!