[Bug]: Custom fee shows ₿1000 for higher than 'normal' sats/vb value when sending near max balance

[catch-21]

What happened?

When sending max (or near max) savings balance, the option to choose custom fee is available but most values calculate ₿1000 fee, which appears to be a default 'fallback' value rather than a valid calculation.

Expected behavior

It should not be possible to increase custom fee sats/vb value when maximum fee amount is reached.
When fee would exceed total input amount (considering limitation of all inputs) then prevent increasing sats/vb.
Could also show an informative toast message to justify the limitation.

Steps to Reproduce

1. Have savings balance
2. Go to send onchain, input onchain address
3. Tap 'Available (Savings)' to apply the maximum (minus 'reverse balance', which is for the fee)'
4. Continue to custom fee selection
5. Observe it is possibly to input any custom sats/vb value and for amounter higher than 'normaly' the amount is always ₿1000

Logs / Screenshots / Recordings

Video first shows normal custom fee calcluation when not sending max, then shows broken calculation when sending max

Screenrecorder-2026-03-26-07-19-28-663.mp4

Bitkit Version

v2.1.2

Device / OS

No response

Reproducibility

Always

Additional context

No response