feat(myopencre): add CSV upload UI with environment-based feature flag

[PRAteek-singHWY]

Summary

Adds the next step of the MyOpenCRE flow by introducing a CSV upload UI for mapping custom standards to OpenCRE.

What’s included

• CSV file upload UI on the MyOpenCRE page
• Client-side validation to allow CSV files only
• Feature flag:
  • ✅ Enabled for local execution
  • ❌ Disabled on hosted OpenCRE (Heroku) to prevent resource abuse
• Clear user messaging when upload is disabled
• No backend or import logic yet (UI scaffold only)

Scope

• Frontend only
• No API changes
• No import processing yet

Context

This PR continues the work started in:

• feat(myopencre): add initial MyOpenCRE page and CSV template download #661 – MyOpenCRE entry point & CSV download
• feat(myopencre): enable CSV download of all CREs #662 – Download full CRE catalogue

This PR intentionally keeps the upload action stubbed; backend import and notifications will follow in subsequent PRs.

Why feature-flagged?

Heroku does not have sufficient resources to safely support full standard imports.
Local execution allows full imports without impacting hosted OpenCRE.

Screenshots

[PRAteek-singHWY]

⚠️ This PR depends on:

• feat(myopencre): enable CSV download of all CREs #662 (CSV download of all CREs)

Please review after the above PR is merged.