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.

internal/services/collection_service.go

@@ -155,6 +155,23 @@ func (s *CollectionService) GetUserCollections(ctx context.Context, userID uuid.
 
 // UpdateCollection updates a collection
 func (s *CollectionService) UpdateCollection(ctx context.Context, collectionID uuid.UUID, name, description, color, icon string, autoAssignRules []Rule, viewSettings map[string]any) (database.Collections, error) {
+	// A2c §3.3: the edit form carries only the toggles it renders — merge
+	// them OVER the stored view_settings so provenance keys
+	// (imported_from_series) and anything else on the row survive.
+	if len(viewSettings) > 0 {
+		existing, err := s.db.GetCollection(ctx, pgtype.UUID{Bytes: collectionID, Valid: true})
+		if err == nil && len(existing.ViewSettings) > 0 {
+			var stored map[string]any
+			if json.Unmarshal(existing.ViewSettings, &stored) == nil {
+				merged := stored
+				for k, v := range viewSettings {
+					merged[k] = v
+				}
+				viewSettings = merged
+			}
+		}
+	}
+
 	// Convert rules to JSONB ([]byte)
 	var rulesJSON []byte
 	if len(autoAssignRules) > 0 {