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/handlers/collections.go

@@ -52,6 +52,12 @@ type UpdateCollectionRequest struct {
 	Icon            string                 `form:"icon" json:"icon"`
 	AutoAssignRules []services.Rule        `form:"auto_assign_rules" json:"auto_assign_rules"`
 	ViewSettings    map[string]interface{} `form:"view_settings" json:"view_settings"`
+	// A2c §3.3: playlist toggles — merged into view_settings server-side
+	// (nil = untouched). []string because the form posts a hidden "false"
+	// sibling next to the checkbox; the LAST value wins. JSON clients may
+	// send a single-element array (or omit — untouched).
+	HideFinished []string `form:"hide_finished" json:"hide_finished"`
+	ShowUpNext   []string `form:"show_up_next" json:"show_up_next"`
 }
 
 type AddBooksRequest struct {
@@ -404,6 +410,23 @@ func (h *CollectionHandler) UpdateCollection(c *echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
 	}
 
+	// A2c §3.3: playlist toggles ride the edit form as top-level fields and
+	// merge into view_settings server-side (the form can't bind a map).
+	// The form posts a hidden "false" sibling before the checkbox, so the
+	// LAST value is the checkbox state.
+	if v := optionalFlag(req.HideFinished); v != nil {
+		if req.ViewSettings == nil {
+			req.ViewSettings = map[string]interface{}{}
+		}
+		req.ViewSettings["hide_finished"] = *v
+	}
+	if v := optionalFlag(req.ShowUpNext); v != nil {
+		if req.ViewSettings == nil {
+			req.ViewSettings = map[string]interface{}{}
+		}
+		req.ViewSettings["show_up_next"] = *v
+	}
+
 	collection, err := h.collectionService.UpdateCollection(
 		c.Request().Context(),
 		collectionID,
@@ -1336,3 +1359,13 @@ func (h *CollectionHandler) ReorderItems(c *echo.Context) error {
 	}
 	return c.JSON(http.StatusOK, map[string]string{"status": "reordered"})
 }
+
+// optionalFlag interprets a multi-valued form flag: last value wins
+// (hidden-false sibling precedes the checkbox), absent → nil (untouched).
+func optionalFlag(values []string) *bool {
+	if len(values) == 0 {
+		return nil
+	}
+	v := strings.EqualFold(values[len(values)-1], "true")
+	return &v
+}