feat: add Web UI artist detail, archive, and ignore actions

Implements Task 8: artist detail page (local albums + found-missing
with ignore buttons), ignored-releases archive with restore, and POST
handlers toggling ignore flags and ignore_singles. Adds ErrArtistNotFound
sentinel so callers can distinguish missing artists, and wires routes via
the enhanced ServeMux path wildcard.

internal/navidrome/sync.go

@@ -2,7 +2,6 @@ package navidrome
 
 import (
 	"context"
-	"database/sql"
 	"errors"
 	"fmt"
 
@@ -111,7 +110,7 @@ func SyncArtists(ctx context.Context, client *NavidromeClient, db *database.DB)
 			settings.Monitored = existing.Monitored
 			settings.IgnoreSingles = existing.IgnoreSingles
 			settings.IgnoreCompilations = existing.IgnoreCompilations
-		} else if !errors.Is(err, sql.ErrNoRows) {
+		} else if !errors.Is(err, database.ErrArtistNotFound) {
 			return fmt.Errorf("sync artists: get settings for artist %s: %w", artist.ID, err)
 		}