Add in-app edit UI for dashboard config

A web editor at /edit for managing pages and widgets without hand-editing YAML:

- Page list and per-page widget views with breadcrumb navigation
- Add/delete pages, add/delete/reorder widgets within a column
- Per-widget textarea editor with curated starter templates, widget-specific
  help (example, full field reference, link to upstream docs), and inline
  error rendering that preserves the user's edits on validation failure
- Pre-save validation via newConfigFromYAML; atomic temp+rename writes with
  .bak backup; YAML edits go through yaml.Node so comments and unresolved
  ${env:X}/${secret:X} tokens survive round-trips
- Auth gate: editing requires auth.users to be configured, or
  admin.allow-without-auth: true as an explicit opt-in for trusted networks
- Pencil button on the dashboard near the theme picker for quick access
- Admin views read fresh from disk so the list updates immediately after
  saves rather than lagging the fsnotify watcher

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

internal/glance/templates/page.html

@@ -43,6 +43,13 @@
                 </div>
             </div>
             {{ end }}
+            {{- if or .App.RequiresAuth .App.Config.Admin.AllowWithoutAuth }}
+            <a class="block self-center" href="{{ .App.Config.Server.BaseURL }}/edit" title="Edit dashboard">
+                <svg class="logout-button" stroke="var(--color-text-subdue)" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5">
+                    <path stroke-linecap="round" stroke-linejoin="round" d="m16.862 4.487 1.687-1.688a1.875 1.875 0 1 1 2.652 2.652L6.832 19.82a4.5 4.5 0 0 1-1.897 1.13l-2.685.8.8-2.685a4.5 4.5 0 0 1 1.13-1.897L16.863 4.487Zm0 0L19.5 7.125" />
+                </svg>
+            </a>
+            {{- end }}
             {{- if .App.RequiresAuth }}
             <a class="block self-center" href="{{ .App.Config.Server.BaseURL }}/logout" title="Logout">
                 <svg class="logout-button" stroke="var(--color-text-subdue)" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5">
@@ -92,6 +99,15 @@
             </div>
             {{ end }}
 
+            {{ if or .App.RequiresAuth .App.Config.Admin.AllowWithoutAuth }}
+            <a href="{{ .App.Config.Server.BaseURL }}/edit" class="flex justify-between items-center">
+                <div class="size-h3">Edit dashboard</div>
+                <svg class="ui-icon" stroke="var(--color-text-subdue)" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24" stroke-width="1.5">
+                    <path stroke-linecap="round" stroke-linejoin="round" d="m16.862 4.487 1.687-1.688a1.875 1.875 0 1 1 2.652 2.652L6.832 19.82a4.5 4.5 0 0 1-1.897 1.13l-2.685.8.8-2.685a4.5 4.5 0 0 1 1.13-1.897L16.863 4.487Zm0 0L19.5 7.125" />
+                </svg>
+            </a>
+            {{ end }}
+
             {{ if .App.RequiresAuth }}
             <a href="{{ .App.Config.Server.BaseURL }}/logout" class="flex justify-between items-center">
                 <div class="size-h3">Logout</div>