Collapse TOC into sticky popover button

[simonhamp]

Summary

• Replaces inline table of contents on plugin and docs pages with a compact Flux dropdown/popover button ("On this page")
• Button sticks to the top-right of the content area as users scroll, with an opaque background for contrast
• Plugin page TOC uses Alpine.js dynamic heading extraction; docs page TOC uses server-side $tableOfContents data
• Adds scrollbar width compensation CSS for modal/popover overlay states (WIP — not fully resolved)

Test plan

• Visit a plugin detail page with a README — verify "On this page" button appears sticky at top-right
• Click the button — verify popover opens with correct heading hierarchy (h2/h3 indentation)
• Click a heading link — verify smooth scroll to the correct section
• Visit a docs page — verify the same TOC popover button appears at top-right of content
• Verify the TOC is no longer shown inline in the sidebar on docs pages
• Test on mobile viewport — verify button is visible and functional
• Test dark mode — verify button and popover have correct styling

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