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/admin-schemas.go

@@ -236,7 +236,15 @@ var widgetSchemas = map[string][]widgetFieldSchema{
 		{Key: "frameless", Label: "Hide widget frame", Type: "boolean"},
 	},
 
-	// group + split-column intentionally skipped: their `widgets:` field is a
-	// recursive list of widget objects, which the inline form generator can't
-	// render usefully yet. They fall through to the YAML editor.
+	"group": {
+		{Key: "title", Label: "Custom title", Type: "string"},
+		{Key: "widgets", Label: "Widgets in group", Type: "list-of-widgets", Required: true,
+			Help: "Each tab inside the group. Nested groups and split-columns aren't allowed."},
+	},
+
+	"split-column": {
+		{Key: "title", Label: "Custom title", Type: "string"},
+		{Key: "max-columns", Label: "Maximum columns (≥2)", Type: "number"},
+		{Key: "widgets", Label: "Widgets shown in the split", Type: "list-of-widgets", Required: true},
+	},
 }