[copyright + license] Add missing headers

Add copyright and license information headers to the source code files
inside the `web` directory and the `capa/render/proto/capa.proto` file.
I have used addlicense to add the headers.

web/explorer/src/assets/main.css

@@ -1,3 +1,19 @@
+/**
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 body {
     margin: 0 auto;
     font-weight: normal;