frontend: Reset current_exercice when viewing theme page

frontend/fic/src/routes/[theme]/+page.svelte

@@ -13,7 +13,10 @@
  import { goto } from '$app/navigation';
 
  import { current_theme } from '$lib/stores/themes';
+ import { set_current_exercice } from '$lib/stores/exercices';
  import { my } from '$lib/stores/my.js';
+
+ set_current_exercice.set(null);
 </script>
 
 <Card class="bg-dark niceborder text-indent mt-2 mb-4">