Implement release slider timeline and dropdown back flow

[unclehowell]

Summary

• replace the top release buttons with a horizontally scrollable range slider that maps each stop to the existing roadmap popups
• wire slider ticks and labels to Featherlight so each selection immediately opens the matching dialog while keeping the active marker highlighted
• keep the cascading dropdown/back control ahead of the slider and refresh its secondary options when the primary value changes

Testing

• not run (not requested)

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Codex Task