feat: add Unix Timestamp Converter tool

Summary

Add a timestamp.html tool that converts Unix timestamps to human-readable dates and vice-versa — one of the most consistently searched developer queries.

Motivation

"Unix timestamp to date" and "epoch converter" are among the top dev-tool searches. The conversion is trivial with the native Date API (no library needed) and the result is immediately useful for debugging logs, JWT exp/iat claims, and API responses.

Proposed UX

• Timestamp → Date panel:
  • Input: epoch in seconds or milliseconds (auto-detected)
  • Outputs: Local datetime, UTC datetime, ISO 8601, relative time ("3 days ago / 2 hours from now")
  • "Now" button fills the input with the current epoch
• Date → Timestamp panel:
  • <input type="datetime-local"> + timezone offset selector
  • Outputs: epoch seconds and epoch milliseconds, both copyable
• Live update as you type (debounced)
• Error state for invalid / out-of-range inputs

Implementation notes

• Category: Code
• Slug: timestamp
• Icon: 🕐
• No npm dependencies — Date API only; relative time via Intl.RelativeTimeFormat
• All logic inline in <script>
• Follow src/_tool-template.html and run bun run generate:sections, bun run generate:index, bun run generate:favicon after adding to tools.json

Start by reading src/_tool-template.html, inspecting existing tools, and reviewing tools.json for the required metadata and generation flow. Add the timestamp tool with both conversion panels, validation, live updates, copyable outputs, and the specified icon and slug, then run bun run generate:sections, bun run generate:index, and bun run generate:favicon; done means the generated indexes and favicon metadata include the new tool.