feat(series): multi-field search, sort-direction toggle, apply placement

Toolbar feedback round: the series search box now matches whatever the
main search matches for a group — series NAME, any book's AUTHOR, and
any book's TITLE inside the series (WHERE EXISTS per group-row, so
counts stay true); the separate author input is gone from the toolbar
(the author= param stays live for the JSON API and deep links). The
sort control becomes a base-key select (Date added/Name/Author/Books/
Progress/Last read) plus a chevron direction toggle that flips and
resubmits; sort/dir compose through ResolveSeriesSort (full registry
keys keep working for back-compat, unknown base keys fall to recent).
The search/apply button moved next to the search box as requested, and
default URLs stay clean (sort/dir only emitted once the user chooses).

internal/router/frontend.go

@@ -151,17 +151,28 @@ func registerFrontendRoutes(cfg *Config) {
 
 		// Series-page state (URL is the source of truth — plain GET form,
 		// no htmx on this page). Unknown values resolve to defaults in the
-		// service's registries, so hand-edited URLs stay safe.
+		// service's registries, so hand-edited URLs stay safe. The URL
+		// carries the BASE sort key + direction; opts.Sort gets the
+		// composed registry key (full keys win on re-resolve). With no
+		// sort param at all, state stays empty so default URLs remain
+		// clean (the toolbar treats empty as recent/desc).
+		rawSort := c.QueryParam("sort")
+		sortKey, sortDir := services.ResolveSeriesSort(rawSort, c.QueryParam("dir"))
 		state := templates.SeriesPageState{
-			Sort:   c.QueryParam("sort"),
+			Sort:   services.SeriesSortBase(sortKey),
+			Dir:    sortDir,
 			Query:  c.QueryParam("q"),
-			Author: c.QueryParam("author"),
 			Status: c.QueryParam("status"),
 		}
+		if rawSort == "" {
+			state.Sort = ""
+			state.Dir = ""
+		}
 		opts := services.SeriesPageOptions{
-			Sort:   state.Sort,
+			Sort:   sortKey,
+			Dir:    sortDir,
 			Query:  state.Query,
-			Author: state.Author,
+			Author: c.QueryParam("author"), // API/deep-link back-compat; the toolbar's multi-field q covers authors
 			Status: state.Status,
 		}
 		if viewerID, vidErr := uuid.Parse(user.ID); vidErr == nil {