Fix sidebar navigation and desktop sponsor layout

[carlospolop]

Summary

• Keep the desktop side sponsor anchored to the bottom of the viewport while the page TOC uses all remaining height and scrolls independently.
• Render exactly one external-link icon in the left navigation.
• Normalize the current URL and expand every ancestor chapter for deep links, including URLs with query parameters or fragments.

Validation

• mdbook build completed successfully.
• theme/toc.js.hbs passed Node syntax validation and both changed files passed git diff --check.
• Ran the book in Docker at localhost:3338.
• Tested a deeply nested AWS page in headless Chrome: exactly one active entry, all three path levels expanded, one external icon, sponsor at the bottom, and no TOC overlap.
• Visually reviewed full desktop and external-link sidebar screenshots from the Docker build.