ux: Implement strict number-only masking for the token amount input

[AlAfiz]

Description

HTML type="number" inputs still allow users to type characters like "e" (exponent), "+", and "-", which breaks our swap math.
If a user pastes text or invalid characters into the amount field, the app calculates NaN and crashes.
We need to implement a strict input mask or regex filter that absolutely prevents anything other than numbers and a single decimal point from being entered.
This bulletproofs the UI against formatting bugs.

Requirements

• Update the onChange handler inside the TokenInput.tsx component.
• Write a regex filter (e.g., /^[0-9]*\.?[0-9]*$/) to validate the raw string value.
• If the new value doesn't match the regex, return early and do not update the React state.
• Ensure users can still cleanly delete numbers and type decimal values like 0.55.