Update compact code dropdown UI

What does this MR do and why?

Describe in detail what your merge request does and why. Please keep this description updated with any discussion that takes place so that reviewers can understand your intent. Keeping the description updated is especially important if they didn't participate in the discussion.

• Add IDE and download sections

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• see this note for design
• second part #450667 (closed). implementation plan for the whole issue is here
• depends on !178890 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

flag On	flag Off

How to set up and validate locally

1. Enable directory_code_dropdown_updates and blob_repository_vue_header_app feature flag

   Feature.enable(:directory_code_dropdown_updates)
   Feature.enable(:blob_repository_vue_header_app)

Related to #450667 (closed) #513599 (closed)