Add API Docs link to the wide navigation rail

Add an "API Docs" navigation entry that opens /api/docs in a new tab.
It is shown only in the wide-mode navigation rail (not the compact
bottom bar) and sits just before About.

Generalises the navigation destination model so an entry can be an
in-app route or an external link, and can be restricted to wide layouts.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/routes.dart

@@ -7,6 +7,10 @@ abstract final class Routes {
   static const String settings = '/settings';
   static const String about = '/about';
 
+  /// Backend-served API documentation. This is not an in-app route: it is opened
+  /// in a new tab, resolved against the configured backend origin.
+  static const String apiDocs = '/api/docs';
+
   /// Path segment for the device-detail route, nested under [devices].
   static const String deviceDetailSegment = ':macAddress';