fix: fee limits for LND and LDK, isolated balance calculation

[rolznz]

There are two isolated app (subwallet) vulnerabilities fixed in this PR:

• use signed integers in isolated app balance calculation to prevent integer overflow
• enforce fee limits when making payments

TESTING:

• LDK: I made 3 nodes, and could pay from A -> C through B, when B sets the routing fee for channel A->B at 10 sats, but fails when B sets the routing fee for channel A->B at 11 sats. Changing back to 10 sats works again. Max fee calculation uses the same code as the transaction service (which is used for isolated apps and budgets).

11 sats:

2025-01-05 08:33:37 TRACE [lightning::routing::router:3230] Ignored 0 candidate hops due to insufficient value contribution, 0 due to path length limit, 0 due to CLTV delta limit, 0 due to previous payment failure, 0 due to htlc_minimum_msat limit, 0 to avoid overpaying, 2 due to maximum total fee limit. Total: 2 ignored candidates.
2025-01-05 08:33:37 ERROR [lightning::ln::outbound_payment:997] Failed to find route for payment with id 8a5ecf53b069559f9c6e318a64fc2984d2f0931101170e60f581e7a1161521e6 and hash 8a5ecf53b069559f9c6e318a64fc2984d2f0931101170e60f581e7a1161521e6
2025-01-05 08:33:37 ERROR [ldk_node::payment::bolt11:160] Failed to send payment: RouteNotFound

10 sats:

2025-01-05 08:35:25 INFO  [lightning::routing::router:3391] Got route: path 0:
 node_id: 0314565c4af998c238a1d286e1feac4e4cd76bc416084096b26d8611f0546e6727, short_channel_id: 1918308041376202752, fee_msat: 10000, cltv_expiry_delta: 72
 node_id: 028052a3659267de42e7e127925594cd5a0cfb2f39563599aff3dc97fc07783d86, short_channel_id: 1918339927213408257, fee_msat: 1000, cltv_expiry_delta: 24
 blinded_tail: None

• LND (same as above, with polar, using lncli updatechanpolicy --base_fee_msat 11000 --chan_point ccd6e82c4d55c0be3a6ff4622a7b213b4aa01f 7ce504b44c1dbce5b306dd5f3a:0 --fee_rate 0 --time_lock_delta 18 the payment cannot be sent, but with lncli updatechanpolicy --base_fee_msat 10000 --chan_point ccd6e82c4d55c0be3a6ff4622a7b213b4aa01f 7ce504b44c1dbce5b306dd5f3a:0 --fee_rate 0 --time_lock_delta 18 the payment can be sent)

• Balances: I manually edited one of my sent transactions on an isolated app to have a huge fee. I correctly see a negative balance and cannot make payments. nwc get_balance and HTTP API return correct value for negative balances.