feat: improve vault form validation

[pntech20]

Summary

• add shared XLM amount validation for deposit and withdraw flows
• show inline helper/error text, invalid field styling, and disabled submit buttons until inputs are valid
• add form tests for invalid values, precision limits, max withdrawal, and atomic-unit submission

Checks

• npm test -- --runInBand
• npm run build
• git diff --check

Closes #16