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/admin.html

@@ -0,0 +1,40 @@
+{{- template "document.html" . }}
+
+{{- define "document-title" }}Edit - {{ .App.Config.Branding.AppName }}{{ end }}
+
+{{- 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>
+        <p class="color-subdue margin-bottom-25">
+            Editing <code>{{ .App.ConfigPath }}</code>. Changes take effect via hot-reload.
+        </p>
+
+        <h2 class="size-h2 margin-bottom-10">Pages</h2>
+        <ul class="list list-gap-10 margin-bottom-25">
+            {{- range .Pages }}
+            <li class="flex items-center gap-10">
+                <div class="grow">
+                    <a class="color-primary" href="{{ $.App.Config.Server.BaseURL }}/edit/pages/{{ .Slug }}">
+                        {{ .Title }}
+                    </a>
+                    <span class="color-subdue"> — {{ .WidgetCount }} widget{{ if ne .WidgetCount 1 }}s{{ end }}</span>
+                    <div class="color-subdue size-h5">/{{ .Slug }}</div>
+                </div>
+                <form method="post" action="{{ $.App.Config.Server.BaseURL }}/edit/api/pages/{{ .Slug }}/delete"
+                      onsubmit="return confirm('Delete page &quot;{{ .Title }}&quot; and all its widgets?');">
+                    <button type="submit" class="color-negative">Delete</button>
+                </form>
+            </li>
+            {{- end }}
+        </ul>
+
+        <h3 class="size-h3 margin-bottom-10">Add a page</h3>
+        <form method="post" action="{{ .App.Config.Server.BaseURL }}/edit/api/pages" class="flex gap-10 items-center">
+            <input type="text" name="title" required placeholder="Page title" class="input grow">
+            <button type="submit">Add page</button>
+        </form>
+    </main>
+    {{ template "footer.html" . }}
+</div>
+{{- end }}