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/config.go

@@ -65,6 +65,12 @@ type config struct {
 		AppBackgroundColor string        `yaml:"app-background-color"`
 	} `yaml:"branding"`
 
+	Admin struct {
+		// AllowWithoutAuth permits access to /admin even when auth.users is empty.
+		// Off by default because admin can rewrite the config file.
+		AllowWithoutAuth bool `yaml:"allow-without-auth"`
+	} `yaml:"admin"`
+
 	Pages []page `yaml:"pages"`
 }