feat(myopencre): enable CSV download of all CREs

[PRAteek-singHWY]

⚠️ Dependency notice

This PR is stacked on top of PR #661 and depends on it.
Please review and merge #661 first.

Summary

This PR implements the first step of the MyOpenCRE flow by enabling users to download a CSV containing all existing CREs via the UI.

Context

The earlier MyOpenCRE work introduced the page and template.
This PR completes the download requirement by returning a CSV populated with all CREs from the database.

Scope

• Adds MyOpenCRE page entry
• Adds a download button in the UI
• Fetches /rest/v1/cre_csv and downloads a valid CSV file

Out of scope

• CSV upload and import
• Feature flagging / Heroku restrictions

These will be handled in follow-up PRs.

How to test

1. Run the app locally
2. Navigate to MyOpenCRE
3. Click Download Mapping Template
4. Verify a CSV file downloads containing CRE data

Closes #584 (partial)

[PRAteek-singHWY]

Hi! This PR depends on #661. I’ve squashed and re-signed the commits so everything is verified.
Happy to rebase once #661 is merged if preferred. Thanks!

[PRAteek-singHWY]

Review focus (apologies for noisy diff)

Apologies for the larger diff than expected — the functional changes in this PR are limited to the following files:

• application/frontend/src/pages/MyOpenCRE/MyOpenCRE.tsx
• application/frontend/src/pages/MyOpenCRE/MyOpenCRE.scss
• application/frontend/src/routes.tsx
• application/frontend/src/scaffolding/Header/header.scss

All other changes are incidental (lockfiles / generated assets) from syncing branches during development and do not affect the CSV download behavior.

Thanks for your patience, and please let me know if you’d prefer me to split or clean up the incidental changes in a follow-up PR.

[PRAteek-singHWY]

This PR is part of the MyOpenCRE frontend flow resolving #584. Full context and next steps are summarized on the issue.