Pipeline Graph Restructure: New Stage Column and CSS Updates

What does this MR do?

This is the third MR in a series of MRs to restructure the main Pipeline Graph (see: #276949 (closed)).

It covers removing the old-style links in the main graph and the CSS adjustments that follow on these changes. It adds in the layout wrapper for the main graph. This will be used both here and by the CI Yaml Visualization (#263141 (closed)) in order to keep both graphs in sync, layout-wise.

The linked column views for upstream and downstream will follow in a subsequent MR.

Much of this work is behind the :graphql_pipeline_details flag, which is off. It does touch a few live components in terms of CSS, though.

Screenshots (strongly suggested)

	main graph	mini-dropdown
flag on
flag off

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Related to #276949 (closed)