Allow naming without an artist folder and per-folder naming rules

The artist folder can be left empty for Album/01 Title (no artist tag
needed), and folders such as Video Games can have their own formats.
Files in a rule folder are only renamed inside it; other files are never
moved into it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -824,7 +824,8 @@ func (s *Server) preAction(w http.ResponseWriter, r *http.Request, pairID int64,
 // ---- organizer ----
 
 func (s *Server) handleNamingInfo(w http.ResponseWriter, r *http.Request) {
-	writeJSON(w, 200, map[string]any{"defaults": engine.DefaultNaming(), "presets": engine.NamingPresets, "tokens": engine.TokenHelp})
+	writeJSON(w, 200, map[string]any{"defaults": engine.DefaultNaming(), "presets": engine.NamingPresets, "tokens": engine.TokenHelp,
+		"folder_default": engine.SoundtrackFolder})
 }
 
 // handleNamingExamples renders sample songs with unsaved settings, using the