fix(ui): show validation feedback for investment amount

[I-lost-everytime]

Problem

When entering an investment amount, invalid values (e.g. zero or empty input) were not clearly communicated to the user. This could lead to confusion during the investment flow, especially for first-time users.

Solution

This PR adds immediate visual validation feedback to the investment amount input by displaying a clear error message when the entered amount is invalid.

What’s changed

Displays an error message when the investment amount is not valid

Uses existing Avalonia bindings (IsValid) for validation state

Error message is styled consistently with system error colors

No changes to business logic or transaction flow

Why this matters

Improves UX by giving users instant, actionable feedback

Reduces friction and mistakes during the investment process

Keeps validation logic simple and UI-only

Scope

UI-only change

No protocol, wallet, or transaction logic affected

No breaking changes

Testing

Manually verified by entering invalid and valid amounts in the investment flow

Error message appears and disappears correctly based on validation state

[dangershony]

Hi thanks for the pr, fyi the ui is undergoing a redesign to V2 so that might effect this PR review

[I-lost-everytime]

Hi thanks for the pr, fyi the ui is undergoing a redesign to V2 so that might effect this PR review

Thanks for the heads-up!

That makes sense — I understand the UI is moving toward V2.
This change is intentionally small and UI-only, using existing bindings (IsValid) without touching any business or transaction logic.

If this conflicts with the V2 direction, I’m totally fine with adjusting it to fit the new design or closing it and reworking the same validation in V2 instead.

Happy to follow whatever makes the most sense for the roadmap 👍

[dangershony]

@SuperJMN please provide feedback