Update license to AGPLv3, add some license headers

src/client/gfx_context.c

@@ -1,4 +1,25 @@
-#include "gfx_context.h"
+/*
+
+  LibreFront --- an OpenFront clone
+  Copyright (C) 2025  CFPI-FPSI
+  Copyright (C) 2025  Emil Kosz
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU Affero General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU Affero General Public License for more details.
+
+  You should have received a copy of the GNU Affero General Public License
+  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+*/
+
+#include <client/gfx_context.h>
 
 GfxContext *
 GfxContext_new ()