refactor: move shared utilities to lib/shared/ sub-packages

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/briefwechsel/CorrespondentSuggestionsDropdown.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
-import { clickOutside } from '$lib/actions/clickOutside';
+import { clickOutside } from '$lib/shared/actions/clickOutside';
 
 interface Correspondent {
 	id: string;