UI: author facet in web sidebar (parity with OPDS)

[deOliveira-R]

The OPDS catalog exposes `/opds/authors` and `/opds/authors/{id}` but the web UI sidebar has no Author facet.

Why

Browsing by author is a primary axis in Zotero. Want the same in the web UI.

Problem to solve

A real library has thousands of authors — can't list them all in the sidebar. Options:

1. Top-N by item count (first pass): show the 20 most-prolific authors in the library; add a "more…" link to a dedicated `/authors` page.
2. Collapsed-by-default section with a filter input at the top ("filter authors…").
3. `/?author=N` filter already needs wiring into `queries._filter_where` (new filter axis).

Acceptance

• `/?author=N` filters the list to items by that author (via `item_authors`).
• Sidebar shows top-20 authors with item counts; active author highlighted.
• Clear-chip on the active-filter row drops just the author axis.
• Combines cleanly with venue / year / type / collection / tag.