feat: consolidate admin features into centralized administration page

- Created administration page at /{recipeLang}/administration accessible only to rezepte_users
- Moved alt-text generator from /admin to /{recipeLang}/admin/alt-text-generator
- Added "Administration" link to user profile dropdown for rezepte_users
- Removed "Unübersetzt" link from main navigation (now accessed via administration page)
- Administration page provides card-based UI with links to:
  - Untranslated Recipes management
  - AI Alt-Text Generator
- Both features now integrated into recipe language routing structure
- Added server-side authentication to all admin routes

src/lib/components/UserHeader.svelte

@@ -1,7 +1,7 @@
 <script lang="ts">
 	import { onMount } from "svelte";
 
-	let { user } = $props();
+	let { user, recipeLang = 'rezepte', lang = 'de' } = $props();
 
 	function toggle_options(){
 		const el = document.querySelector("#options")
@@ -141,6 +141,9 @@ h2 + p{
 			<h2>{user.name}</h2>
 			<p>({user.nickname})</p>
 			<ul>
+				{#if user.groups?.includes('rezepte_users')}
+					<li><a href="/{recipeLang}/administration">Administration</a></li>
+				{/if}
 				<li><a href="https://sso.bocken.org/if/user/#/settings" >Einstellungen</a></li>
 				<li><a href="/logout" >Log Out</a></li>
 			</ul>