Style navigation and spaces with a dark rail and selection markers

[ymote]

Give the desktop navigation rail and mobile navigation/space bars a navy background, with light icons and a teal selection marker. Coordinate space avatars, dividers, and hover states, and widen the navigation area to 76 px.

The selection shader clears the rounded background shape before drawing the accent bar, keeping the accent confined to the edge. Avatar fallback text uses a darker teal background for contrast.

Review: switch between Home and spaces, hover navigation buttons, and compare desktop and mobile layouts.

Extracted from #1068 as a UI-only change. Based directly on current upstream main; no dependency on the other split PRs and no Hagency code.

Validation

• cargo clippy --locked --workspace --all-features --all-targets -- -D warnings
• typos --config .github/typos.toml and git diff --check
• Combined integration: cargo test --locked --lib --all-features — 104 passed.
• Runtime fixture: visually checked the dark rail, selection marker, and mobile bottom navigation.