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/handlers/series.go

@@ -68,6 +68,7 @@ func (h *SeriesHandler) GetSeries(c *echo.Context) error {
 	// unknown values resolve to defaults inside the service.
 	opts := services.SeriesPageOptions{
 		Sort:   c.QueryParam("sort"),
+		Dir:    c.QueryParam("dir"),
 		Query:  c.QueryParam("q"),
 		Author: c.QueryParam("author"),
 		Status: c.QueryParam("status"),