feat(myopencre): add initial MyOpenCRE page and CSV template download

[PRAteek-singHWY]

Summary

This PR introduces the first step of the MyOpenCRE frontend as described in issue #584.

It adds a dedicated MyOpenCRE page that allows users to download a CSV mapping template (header-only), enabling them to map their own security standards (e.g. SOC2) to OpenCRE Common Requirements without using curl or CLI tooling.

What’s included

• New MyOpenCRE frontend page (React + TypeScript)
• Route and navigation entry for /myopencre
• Downloadable CSV mapping template (header-only) defining the expected column structure:
  • standard_name
  • standard_section
  • cre_id
  • notes
• No backend changes

What’s intentionally out of scope

• CSV upload functionality
• Feature flag handling (local vs Heroku)
• Import notifications

These will be handled in follow-up incremental PRs to keep the review surface small.

Related issue

Closes part of #584

Screenshots

• MyOpenCRE navigation entry

• MyOpenCRE landing page]

• Downloaded CSV mapping template