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/engine/organize.go

@@ -29,11 +29,10 @@ import (
 // normalizeOrganize validates a pair's organize settings.
 func normalizeOrganize(p *store.Pair) error {
 	if len(p.OrganizeConfig) > 0 && string(p.OrganizeConfig) != "{}" && string(p.OrganizeConfig) != "null" {
-		var c NamingConfig
-		if err := json.Unmarshal(p.OrganizeConfig, &c); err != nil {
+		c, err := parseNaming(p.OrganizeConfig)
+		if err != nil {
 			return fmt.Errorf("naming settings: %w", err)
 		}
-		fill(&c, DefaultNaming())
 		if err := ValidateNaming(c); err != nil {
 			return err
 		}
@@ -171,8 +170,8 @@ func (e *Engine) planOrganize(ctx context.Context, j *Job, l *libs, sides []stri
 				item.Reason = "file type without editable tags"
 			} else if fields, err := e.tagCache.get(ctx, e.tagTools, filepath.Join(root.Path, rel), in); err != nil {
 				item.Reason = "tags could not be read"
-			} else if to, missing := renderPath(naming, filepath.Ext(rel), fields); missing != "" {
-				item.Reason = "missing " + missing + " tag"
+			} else if to, why := renderPath(naming, rel, fields); why != "" {
+				item.Reason = why
 			} else if to == rel {
 				plan.Conforming++
 				continue