Fix API docs link when app is served from the backend

The API docs nav entry uses the origin-relative path "/api/docs", which
Uri.parse left scheme- and host-less, so canLaunchUrl/launchUrl could not
open it (e.g. in Docker). Resolve external links against the current page
so they carry a scheme and host; absolute URLs pass through unchanged.

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

frontend/lib/navigation.dart

@@ -374,8 +374,15 @@ void _onDestinationSelected(_NavDest destination, BuildContext context) {
 // Opens an external link in a new tab. Origin-relative paths (e.g. the API docs)
 // resolve against the current host, since the backend serves both the front-end
 // and the API docs from the same origin.
+// Resolves an external link against [base] (defaulting to the current page).
+// Origin-relative paths (e.g. "/api/docs") gain the current scheme and host so
+// the resulting URI is launchable; absolute URLs are returned unchanged.
+// Without this, launching a scheme-less URI fails when the app is served from
+// the backend (e.g. Docker).
+Uri resolveExternalUri(String url, {Uri? base}) => (base ?? Uri.base).resolve(url);
+
 Future<void> _openExternal(String url) async {
-  final uri = Uri.parse(url);
+  final uri = resolveExternalUri(url);
   if (await canLaunchUrl(uri)) {
     await launchUrl(uri, mode: LaunchMode.externalApplication);
   }

frontend/test/unit/resolve_external_uri_test.dart

@@ -0,0 +1,28 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/navigation.dart';
+
+void main() {
+  final origin = Uri.parse('http://192.168.1.10:8080/web/devices');
+
+  test('resolves an origin-relative path against the current host', () {
+    expect(
+      resolveExternalUri('/api/docs', base: origin),
+      Uri.parse('http://192.168.1.10:8080/api/docs'),
+    );
+  });
+
+  test('preserves the scheme and host of an https origin', () {
+    final secure = Uri.parse('https://example.com/web/');
+    expect(
+      resolveExternalUri('/api/docs', base: secure),
+      Uri.parse('https://example.com/api/docs'),
+    );
+  });
+
+  test('returns an absolute URL unchanged', () {
+    expect(
+      resolveExternalUri('https://docs.example.com/spec', base: origin),
+      Uri.parse('https://docs.example.com/spec'),
+    );
+  });
+}