Panel: edit-page sidebar scrolls with the page and pins when its end is in view

[glennjacobs]

Summary

On the edit pages the right-hand column was position: sticky; top: 60px. When the column is taller than the viewport that pins it at the top and it only scrolls once the main column reaches its end, so cards lower down (including any an add-on injects into the sidebar zones) go unseen.

This adds a StickySidebar component and uses it on all seven edit pages (products, variants, brands, collections, product types, customers, product options):

• When the column fits in the viewport it pins below the breadcrumb bar exactly as before.
• When it is taller, its sticky top is set to the negative overflow, so the column scrolls with the page straight away and pins when its last card reaches the bottom of the viewport.

The height is measured live with a ResizeObserver (plus window resize), so nothing is hard-coded and cards added by add-ons at runtime are accounted for. The component is exported from @lunarphp/panel for add-on pages.

Split out of #2732 at review; #2732 will rebase onto this.

Test plan

• npx vitest run in packages/panel (343 passed, 3 new cases: fits, overflows, resize)
• npm run type-check, npm run build (compiled bundle committed)
• vendor/bin/pest tests/panel/Unit/PageScaffoldTest.php
• Manual: open a product with a long sidebar, scroll, confirm the column moves immediately and stops at its end

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