dropping timetrack variable on logout

Signed-off-by: Stephan Richter <s.richter@srsoftware.de>

frontend/src/user.svelte.js

@@ -29,6 +29,7 @@ export async function logout(){
     await fetch(url,{
         credentials: 'include'
     });
+    timetrack = null;
     user.name = null;
 }