[Feature] Add timestamp converter

[yeleepark]

Summary

Add a browser-based timestamp converter for translating Unix timestamps into readable dates and converting dates back into timestamps.

Why

Timestamp conversion is a common utility task for developers, support teams, and anyone checking logs, API payloads, scheduled jobs, or analytics data. It expands oddkit beyond image tools while staying lightweight, practical, and local-first.

Proposed scope

• Convert Unix timestamps to human-readable dates
• Convert date/time input back to Unix timestamps
• Support seconds and milliseconds
• Show values in local time and UTC
• Allow selecting or comparing time zones if feasible
• Copy generated values to the clipboard
• Process everything locally in the browser

UX notes

• Keep input and output visible at the same time
• Use mono typography for timestamps and generated values
• Suggested microcopy: // what time is 1720000000

Acceptance criteria

• Seconds and milliseconds timestamps are parsed correctly
• Date input can be converted back into timestamp values
• UTC and local time outputs are clearly labeled
• Invalid input shows a useful error state without crashing
• No input data is uploaded to a server