Refine release timeline slider and layout

[unclehowell]

Summary

• rebuild the release timeline markup in both index pages so the slider track, datalist, and stage labels are statically rendered and visible under the fixed navigation
• update the release slider scripting to work with the new markup, clamp selections, set aria value text, and support keyboard activation while still launching the existing popups

Testing

• not run (not requested)

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Codex Task