max spendable amount should adjust with wallet balance

[callebtc]

Example:

Max amount set to 200k, initial wallet balance at 1M, everybody sees 200k... as I use the wallet, balance drops to 150k, now max amount is shown as 150k

Note: Max amount should take fee_reserve into account!

Optional check box in LNURL settings "dynamic max amount" (warning: terminal will see your balance if it falls below this number).