1.3.2 Input validation: only allow decimal values (example regex: ^(0|[1-9]\d*)(\.\d+)?$) #8

Background

Currently the user can type any characters into the amount input field, which may lead to unexpected display issues.

Goal

Restrict the input so that only valid numeric decimal values are allowed. When the user attempts to enter invalid characters, show a gentle inline warning.

Definition of Done

• The field accepts only decimal numbers (example regex: ^(0|[1-9]\d*)(\.\d+)?$). Scientific notation is not allowed.
• Invalid characters are blocked on type and on paste, including IME input cases.
• A gentle inline hint appears when input is invalid. No modal is used.
• The component preserves caret position and common editing keys continue to work.
• Screen reader users are informed via an inline, aria-live announcement.
• Unit tests cover valid and invalid cases, typing, pasting, and hint visibility.
• README is updated under “Validation & Precision” to document the rules.