Feature: glide the docs navigation and animate disclosures

[SandObserver]

Summary

• Add a moving hover pill to the sidebar links and group headers, and a moving current-section pill to the "On this page" list.
• Slide the sidebar current-page highlight between pages with a cross-document view transition on desktop widths.
• Animate the height and content opacity of sidebar groups, changelog entries and service list rows on open and close, without changing Starlight's saved sidebar state.
• Disable all of the above under prefers-reduced-motion: reduce.
• Add top padding to expanded service list details.
• Centre the code block "Copied!" tooltip on the copy button.

Testing
All green. Checked in Chrome in light and dark colour schemes, including reduced motion.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.