Round out the editor: head widgets, undo, group/split-column, site settings

- Head widgets are now first-class in dashboard edit mode: they get
  drag/edit/delete handles, their own SortableJS group (so they can't
  cross into columns), and the layout endpoint accepts a
  headWidgets[] sequence alongside columns[].
- New POST /edit/api/restore swaps glance.yml ↔ glance.yml.bak —
  effectively undo (and click again to redo). Surfaced as a "Restore
  previous version" button on /edit.
- group and split-column are now editable inline via a new
  list-of-widgets schema type. Each item picks a widget type from a
  dropdown (excluding group/split-column to honor the backend's
  no-nesting rule) and renders that type's schema-driven sub-form.
  Switching the type re-renders the body.
- New /edit/site-settings page with a form for branding (app name,
  logo, favicon, footer, app icon/background). POST applies via the
  same surgical yaml.Node update used for widgets. Theme/server
  intentionally still YAML for now.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/glance/glance.go

@@ -455,6 +455,7 @@ func (a *application) server() (func() error, func() error) {
 	mux.HandleFunc("GET /edit", a.handleAdminIndex)
 	mux.HandleFunc("GET /edit/pages/{page}", a.handleAdminPage)
 	mux.HandleFunc("GET /edit/pages/{page}/settings", a.handleAdminPageSettings)
+	mux.HandleFunc("GET /edit/site-settings", a.handleAdminSiteSettings)
 	mux.HandleFunc("GET /edit/pages/{page}/widgets/{col}/{idx}", a.handleAdminWidget)
 
 	mux.HandleFunc("POST /edit/api/pages", a.handleAdminAddPage)
@@ -477,6 +478,8 @@ func (a *application) server() (func() error, func() error) {
 	mux.HandleFunc("POST /edit/api/pages/{page}/columns/{col}/size", a.handleAdminColumnSize)
 	mux.HandleFunc("POST /edit/api/pages/{page}/fields", a.handleAdminUpdatePageFields)
 	mux.HandleFunc("POST /edit/api/pages/{page}/move", a.handleAdminMovePage)
+	mux.HandleFunc("POST /edit/api/restore", a.handleAdminRestore)
+	mux.HandleFunc("POST /edit/api/site-settings", a.handleAdminUpdateSiteSettings)
 
 	if a.RequiresAuth {
 		mux.HandleFunc("GET /login", a.handleLoginPageRequest)