Add local glTF viewer and visual coverage

Amp-Thread-ID: https://ampcode.com/threads/T-01a02d1c-27fc-7761-9f51-ed97b2dd4e3e
Co-authored-by: Heaust Azure <heaust.azure@gmail.com>

server/app_test.go

@@ -144,3 +144,24 @@ func TestSavedPlaygroundPageRoute(t *testing.T) {
 		t.Fatalf("content type = %q", contentType)
 	}
 }
+
+func TestGltfViewerPageRoute(t *testing.T) {
+	request := httptest.NewRequest(http.MethodGet, "/view/gltf", nil)
+	response := httptest.NewRecorder()
+	testApplication(t).routes().ServeHTTP(response, request)
+	if response.Code != http.StatusOK {
+		t.Fatalf("status = %d", response.Code)
+	}
+	if contentType := response.Header().Get("Content-Type"); contentType != "text/html; charset=utf-8" {
+		t.Fatalf("content type = %q", contentType)
+	}
+	for _, content := range []string{
+		`"@yawn/core": "/pkg/core.js"`,
+		`"@yawn/handles": "/pkg/handles.js"`,
+		"Drop a glTF file",
+	} {
+		if !bytes.Contains(response.Body.Bytes(), []byte(content)) {
+			t.Fatalf("viewer page does not contain %q", content)
+		}
+	}
+}