feat(filters+ui): per-element author/publisher filters, playlist toggles, Up Next API, reorder UI, dashboard badges

Phases 4 + 5 of the metadata overhaul (plan §4, §5) plus §3 handler
wiring.

Filters (§4.1): author_filter/publisher_filter now match per-element
against the authors[]/publishers[] arrays — a co-authored book matches
any one of its authors — with comma-joined params acting as
OR-within-facet multi-select (string_to_array + cross unnest). Legacy
single-value params unchanged on the wire; series/genre/language
filters stay on their varchar columns; narrator filter already
per-element.

Collections API (§3.3-3.5): collection detail carries viewer +
hide_finished into the listing query and exposes up_next (visible-
library-enforced reading-queue head); PUT
/api/collections/:id/items/order (owner-only, full-order rewrite);
UpdateCollectionRequest gains hide_finished/show_up_next ([]
string last-value-wins for the hidden-false-sibling checkbox pattern)
merged server-side OVER stored view_settings so imported_from_series
provenance survives edits; EditCollectionModal renders the toggles
from view_settings.

Dashboard (§5.5): sections JSON carries is_playlist/complete
(dashboard.ts is the AGENTS-documented JSON-render exception; first
paint stays templ). Fully-read playlists render '✓ Playlist complete'
instead of an empty rail; imported playlists get a 📖 Playlist badge.

Frontend: book detail shows subtitle + full authors + Explicit badge;
edit modal gains subtitle, comma-separated authors/publishers (client-
split to arrays exactly like narrators), step=any series number,
explicit checkbox; collection book cards get playlist move ↑/↓ buttons
persisting the full order via the reorder endpoint (templ partial
re-render on failure — DOM order is client-local, API order is truth).

AGENTS.md gotcha #3 amended per plan §3.7: user collections are
dashboard sections by default; the show_on_dashboard premise is
retired.

Verified: full affected integration sweep green (pins widened for
up_next/is_playlist/complete — changelog entries in
docs/contract_pins.md); unit suite green; go build + templ + vite
green.

templates/book_detail.templ

@@ -63,7 +63,14 @@ templ BookDetail(user User, book handlers.MediaDetail, errorMessage string, tomb
 						<!-- Details -->
 						<div class="flex-1 min-w-0">
 							<h1 class="text-3xl md:text-4xl font-bold tracking-tight mb-2" style="color: var(--text-primary)">{ book.Title }</h1>
-							if book.Author.Valid && book.Author.String != "" {
+							if book.Subtitle.Valid && book.Subtitle.String != "" {
+								<p class="text-lg mb-2" style="color: var(--text-secondary)">{ book.Subtitle.String }</p>
+							}
+							if len(book.Authors) > 0 {
+								<h2 class="text-lg md:text-xl mb-5" style="color: var(--text-secondary)">
+									by { strings.Join(book.Authors, ", ") }
+								</h2>
+							} else if book.Author.Valid && book.Author.String != "" {
 								<h2 class="text-lg md:text-xl mb-5" style="color: var(--text-secondary)">
 									by { book.Author.String }
 								</h2>
@@ -184,6 +191,9 @@ templ BookDetail(user User, book handlers.MediaDetail, errorMessage string, tomb
 							}
 							<!-- Badges: series, direction, tags -->
 							<div class="flex flex-wrap items-center gap-2 mb-3">
+								if book.Explicit {
+									<span class="badge" style="border: 1px solid var(--border); color: var(--text-secondary);" title="Marked explicit in source metadata">Explicit</span>
+								}
 								if book.Series.Valid && book.Series.String != "" {
 									<a
 										href={ "/series/detail?name=" + url.QueryEscape(book.Series.String) + "&library_id=" + uuidToString(book.LibraryID) }