[Enhancement] List view missing for ?group=authors and ?group=series

[kevinheneveld]

Summary

The audiobooks toolbar exposes a list/grid view toggle, but the grouped views (?group=authors and ?group=series) only render as a grid. Toggling to list mode while grouped is silently ignored — the toolbar icon flips, but the page keeps rendering grid cards.

Root cause: the list-mode template in fe/src/views/library/AudiobooksView.vue is nested inside the groupBy === 'books' branch. The grouped-view branch has no viewMode === 'list' rendering at all.

Steps to reproduce

1. Open the audiobooks library.
2. Click the toolbar list/grid toggle so the icon shows list mode.
3. Open the grouping dropdown and switch to Authors (or Series).
4. The view stays as a grid of cards. The list-mode toggle has no effect for grouped views.

Expected behavior

List mode should render a row-per-collection list (cover thumb, collection name, book count). Clicking a row should navigate to the collection page, matching how the grid cards behave.

Notes

Re-opened here from kevinheneveld#1 per discussion on #585 — proposed fix is in that PR.