fix: block renaming main project variant; dynamic page titles

- Add ErrCannotRenameMainVariant; ProjectService.Update now returns
  this error if the caller tries to change the Variant of a main
  project (empty Variant) — ensures there is always exactly one main
- Handle ErrCannotRenameMainVariant in PUT /api/projects/:uuid with 400
- Set document.title dynamically from breadcrumb data:
  - Configurator: "CODE / variant / Config name — QuoteForge"
  - Project detail: "CODE / variant — QuoteForge"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/services/project.go

@@ -21,6 +21,7 @@ var (
 	ErrProjectCodeExists       = errors.New("project code and variant already exist")
 	ErrCannotDeleteMainVariant = errors.New("cannot delete main variant")
 	ErrReservedMainVariant     = errors.New("variant name 'main' is reserved")
+	ErrCannotRenameMainVariant = errors.New("cannot rename main variant")
 )
 
 type ProjectService struct {
@@ -108,7 +109,12 @@ func (s *ProjectService) Update(projectUUID, ownerUsername string, req *UpdatePr
 		localProject.Code = code
 	}
 	if req.Variant != nil {
-		localProject.Variant = strings.TrimSpace(*req.Variant)
+		newVariant := strings.TrimSpace(*req.Variant)
+		// Block renaming of the main variant (empty Variant) — there must always be a main.
+		if strings.TrimSpace(localProject.Variant) == "" && newVariant != "" {
+			return nil, ErrCannotRenameMainVariant
+		}
+		localProject.Variant = newVariant
 		if err := validateProjectVariantName(localProject.Variant); err != nil {
 			return nil, err
 		}