bug: spending balance kept after a full withdrawal #1849

Found in @pwltr's review of #1697 at 5953744 (review).

Behaviour

While Spark reports a zero balance together with a sync error, SparkRepo.refresh keeps showing the last Spending balance (keptBalanceSats), so a sync failure doesn't flash 0. After a withdrawal that spent the whole Spending balance, a sync error in the same refresh keeps the old balance on screen even though it was spent. Sending still uses Spark's actual balance, so nothing can be overspent; only the shown balance is stale until a sync succeeds.

Open

• Confirm the case on a device: withdraw the full Spending balance to Savings, then make the next sync fail.
• If it reproduces, drop the kept balance once a withdrawal of it succeeded.