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/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'),
+    );
+  });
+}