[Feature] Timezone Conversion Checker for Scheduled Message Entries

[phertyameen]

Closes #299

Implements a CLI checker that verifies timezone correctness for entries in schedules.json.

Changes

• Added app/lib/checkTimezones.js — zero-dependency Node.js checker using native Intl APIs
• Added app/data/schedules.json — mock schedule entries covering real-world edge cases
• Updated package.json with check:timezones script
• Added timezone-report.json to .gitignore

What it detects

• Invalid IANA timezone labels (e.g. America/NewYork, UTC+5)
• Non-existent local times caused by DST spring-forward gaps
• Ambiguous local times in DST fall-back overlap windows

Output

• Colour-coded terminal report per entry (VALID / WARNING / ERROR)
• Normalized UTC timestamp for each valid schedule
• Machine-readable timezone-report.json saved to project root

Usage

npm run check:timezones

Notes

• No external dependencies, uses Intl.DateTimeFormat only
• Exits with code 1 if any errors are detected, making it CI-friendly