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

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

frontend/src/hooks.server.ts

@@ -3,7 +3,7 @@ import { paraglideMiddleware } from '$lib/paraglide/server';
 import { sequence } from '@sveltejs/kit/hooks';
 import { env } from 'process';
 import { cookieName, cookieMaxAge } from '$lib/paraglide/runtime';
-import { detectLocale } from '$lib/server/locale';
+import { detectLocale } from '$lib/shared/server/locale';
 
 const PUBLIC_PATHS = ['/login', '/logout', '/forgot-password', '/reset-password', '/register'];