Rework desktop and mobile navigation UI

Replace the floated desktop navigation with a fixed flex-based header and a matching body spacer so page content remains positioned below the header.

Add a mobile bottom bar for primary page actions, including scroll-to-top, search, and menu controls, with pointer-aware hover and touch active states. Hide the bottom bar when it overlaps the footer unless the page height is too short to render the bottom bar at all.

Add an explicit Home link to the shared navigation instead of relying on the site title alone, since treating the title as the only homepage path makes the site hierarchy less clear.

Introduce a shared navigation modal shell for mobile menu interactions and future navigation actions.

Replace the text search entrypoint with an inline SVG icon and restyle the existing search modal for the dark theme.

Author: ritorhymes