Fix sidebar sub-nav items not reacting to hover

[Paelsmoessan]

Summary

In the left sidebar, the sub-nav items (Books/Authors/Series under Audiobooks, and the Settings
children) did not react to hover. You had to hover the parent top node first before a child would
respond, which read as broken.

Changes

Fixed

• Sub-items had no hover or focus feedback. .nav-subitem had CSS for only its default and
  .active states, so hovering (or keyboard-focusing) a sub-item produced no visual change. Added a
  :hover / :focus-visible highlight that matches the existing blue accent.
• Children were unreachable until the parent was hovered. .nav-sub was collapsed with
  pointer-events: none until it received .open (driven by hovering the parent). So for any section
  you were not currently on, the children could not be hovered at all until you first hovered the top
  node. Made the sub-navs stay expanded so children are always interactive and highlight on their own
  hover.

Testing

• Frontend builds clean.
• Verified live: hovering any sub-item (in the active section or any other) now highlights it
  directly, with no need to hover the parent first. Keyboard focus highlights too.

Notes

• The second change removes the collapse/expand animation in favor of an always-expanded tree, since
  that collapse was the source of the reachability gate. If maintainers prefer to keep the accordion,
  I am happy to instead fix reachability another way (for example expanding on child focus/hover
  without the pointer-events block). Flagging it since it is a small behavior change.
• Suggested version label: patch.