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).

templates/types.go

@@ -149,11 +149,12 @@ type SeriesCardData struct {
 
 // SeriesPageState is the /series page's sort/filter state, echoed back
 // into the toolbar controls and the pagination URLs (the URL is the
-// single source of truth on this plain-GET page).
+// single source of truth on this plain-GET page). Sort holds the BASE
+// toolbar key (name/author/…); Dir is the effective direction.
 type SeriesPageState struct {
 	Sort   string
+	Dir    string
 	Query  string
-	Author string
 	Status string
 }