make main landing page bilingual and eliminate language switcher flicker

Replace window.location.reload() with custom event dispatching to avoid
flicker when switching languages on main page. Add bilingual labels for
all content including welcome message and link grid.

src/routes/(main)/+layout.svelte

@@ -1,16 +1,14 @@
 <script>
 import Header from '$lib/components/Header.svelte'
 import UserHeader from '$lib/components/UserHeader.svelte';
-export let data
-let user;
-if(data.session){
-	user = data.session.user
-}
+let { data } = $props();
+
+let user = $derived(data.session?.user);
 </script>
 
 <Header>
 	<ul class=site_header slot=links>
 	</ul>
-	<UserHeader {user} slot=right_side></UserHeader>
+	<UserHeader {user} slot=right_side showLanguageSelector={true}></UserHeader>
 	<slot></slot>
 </Header>