fix: tighten amount input validation

[omerbek]

Summary

Fixes #467.

• Move the shared amount validation rules into a small testable helper.
• Require the whole amount string to match a decimal currency format with an escaped decimal point.
• Reject malformed values such as 12a34, 12,34, and 1.2.3 instead of accepting them through partial parseInt / parseFloat parsing.
• Keep valid values like 12, 12.3, and 12.34 accepted, while still rejecting zero as non-positive.

Testing

corepack yarn install --frozen-lockfile
# failed locally because this host is Node 21.6.1 and nuxt@3.18.1 requires ^20.19.0 || >=22.12.0

corepack yarn install --frozen-lockfile --ignore-engines
# passed; nuxt prepare completed

corepack yarn test components/amountValidation.test.ts --runInBand
# passed, 7 tests

corepack yarn eslint components/AmountInput.vue components/amountValidation.ts components/amountValidation.test.ts
# passed

git diff --check
# passed

Full corepack yarn lint on this Windows checkout still fails on existing repository-wide CRLF/Prettier findings unrelated to this patch, so I also ran scoped eslint on the touched files.