[Feature]: Add maximum bound validation to CJIT flow

[catch-21]

Describe the problem

Since increasing the maximum amount that can added to the spending balance via CJIT to 950euro (as part of the new Liquidity Management Policy for Lightning Channels) it is possible to exceed the boundary so that blocktank rejects it, showing an unpleasing toast error and a loading spinner that displays indefinitely.

Screen-2024-12-09-164136.mp4

Describe the solution

We should add some validation to prevent the user from proceeding when the maximum is exceeded.
Or at least show a nice error and allow the user to retry instead of showing the spinner for eternity.

Additional context

No response

[catch-21]

Verified in v1.0.8
Continue button is disabled when a value higher than the max is entered. Works with and without existing LN liquidity. No informative message displayed.