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/templates/admin.html

@@ -5,7 +5,12 @@
 {{- define "document-body" }}
 <div class="flex flex-column body-content">
     <main class="content-bounds" style="padding-block: 2rem;">
-        <h1 class="size-h1 margin-bottom-15">Edit</h1>
+        <div class="flex items-center gap-10">
+            <h1 class="size-h1 grow">Edit</h1>
+            <a href="{{ .App.Config.Server.BaseURL }}/edit/site-settings">
+                <button type="button">Site settings</button>
+            </a>
+        </div>
         <p class="color-subdue margin-bottom-25">
             Editing <code>{{ .App.ConfigPath }}</code>. Changes take effect via hot-reload.
         </p>
@@ -43,6 +48,17 @@
             <input type="text" name="title" required placeholder="Page title" class="input grow">
             <button type="submit">Add page</button>
         </form>
+
+        <hr style="margin-block: 2rem; border: none; border-top: 1px solid var(--color-widget-content-border);">
+        <h3 class="size-h3 margin-bottom-10">Undo last save</h3>
+        <p class="color-subdue margin-bottom-10">
+            Restores the previous contents of <code>{{ .App.ConfigPath }}</code> from the
+            <code>.bak</code> file written on each save. Click again to redo.
+        </p>
+        <form method="post" action="{{ .App.Config.Server.BaseURL }}/edit/api/restore"
+              onsubmit="return confirm('Restore the previous version of your config?');">
+            <button type="submit">Restore previous version</button>
+        </form>
     </main>
     {{ template "footer.html" . }}
 </div>