Fix topic sidebar toggle styling

[IEvangelist]

Summary

• keep topic landing-page collapse and expand icons transparent and borderless when no On this page bar is present
• preserve the existing bar styling on API reference and topic pages that render the mobile TOC

Validation

• verified the landing page and regular topic page at affected viewport widths using the local dev server and playwright-cli

[Copilot]

Pull request overview

This PR adjusts the “floating” sidebar collapse/expand toggle styling in Sidebar.astro so that topic landing pages (where the mobile “On this page” TOC bar is not present) don’t render the toggle as a leftover TOC bar fragment, while preserving the existing in-bar styling on pages that do render the mobile TOC bar (and keeping API reference behavior intact).

Changes:

• Refines the no-TOC, 50–99.999rem breakpoint styling by splitting “in-bar” visuals by toggle ID (#sidebar-* vs #topic-sidebar-*).
• Keeps the shared positioning behavior for both toggles, while making topic toggle backgrounds/borders transparent when no TOC bar is present.
• Adds explicit hover handling for topic toggles in the no-TOC scenario to prevent reintroducing a bar-like background.

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[aspire-repo-bot]

Frontend HTML artifact ready

The latest frontend build uploaded the frontend-dist artifact for PR #1370. Use the VS Code button below to open this PR with GitHub Artifacts Explorer and browse the built HTML locally.

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