bug: no-route toast when only the spark leg fee doesn't fit #1285

Found in @coreyphillips's review of #1191 (thread). The Android twin is synonymdev/bitkit-android#1850.

Behaviour

An on-chain-only contact paid from Spending gets "Can't pay this contact" when the amount fits the balance but the amount plus Spark's withdrawal fee does not.

• Amount accepts up to the Lightning sendable amount.
• SparkManager.withdrawalRouteLeg quotes no leg when the balance can't cover the amount plus the withdrawal fees.
• So the router finds no route, and the toast blames the contact instead of showing the short-balance copy. The payment itself is correctly refused.

Open

• The router reports no leg it skipped, so the app can't tell a fee shortfall from a contact it can't pay.
• Fix options: count a Spark leg that was quoted but couldn't be covered as a short balance, or cap Amount at Spark's sendable amount minus the leg fee.