Use deterministic UTC parsing for birth dates

[jordanhom]

Motivation

• Prevent timezone-dependent validation and calculation errors by avoiding local-time Date parsing and getTimezoneOffset adjustments.

Description

• In handleCalculation (in js/ui.js) replace the local-Date + getTimezoneOffset normalization with deterministic UTC parsing by splitting the input and calling new Date(Date.UTC(year, monthIndex, day)).
• Validate the parsed date by comparing parsed year/month/day components to the created UTC Date to catch invalid inputs (e.g. 2021-02-30).
• Compare the parsed birth date against a UTC "today" computed with Date.UTC to ensure the date is strictly in the past.

Testing

• No automated tests were run as part of this change.

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Codex Task