templates: make template names actually be filled on creation

templates: fix data race on just returning the list of names, instead we
make a new slice when filling in names

mocks: add missing Executor method that we added in previous commit

templates/loader.go

@@ -60,27 +60,33 @@ func (s *Site) Reload() error {
 		return errors.E(op, err)
 	}
 	s.themes = themes
+	s.populateNames()
 
+	return nil
+}
+
+type TemplateSelector interface {
+	Template(theme, page string) (*template.Template, error)
+}
+
+func (s *Site) Executor() Executor {
+	return newExecutor(s)
+}
+
+func (s *Site) populateNames() {
 	// populate the theme name lists, one for public, one for admin
-	names := maps.Keys(themes)
+	names := maps.Keys(s.themes)
 	slices.Sort(names)
 
+	s.themeNamesAdmin = make([]string, 0, len(s.themeNamesAdmin))
+	s.themeNamesPublic = make([]string, 0, len(s.themeNamesPublic))
 	for _, name := range names {
 		if strings.HasPrefix(name, ADMIN_PREFIX) {
 			s.themeNamesAdmin = append(s.themeNamesAdmin, name)
 		} else {
 			s.themeNamesPublic = append(s.themeNamesPublic, name)
 		}
 	}
-	return nil
-}
-
-type TemplateSelector interface {
-	Template(theme, page string) (*template.Template, error)
-}
-
-func (s *Site) Executor() Executor {
-	return newExecutor(s)
 }
 
 func (s *Site) ThemeNames() []string {
@@ -201,6 +207,7 @@ func FromFS(fsys fs.FS) (*Site, error) {
 	if err != nil {
 		return nil, errors.E(op, err)
 	}
+	tmpl.populateNames()
 
 	return &tmpl, nil
 }