use bindable instead of callback on signup version select

src/app/website/signup/EditionSelection.svelte

@@ -1,10 +1,9 @@
 <script lang="ts">
   interface Props {
-    onSelect: (edition: 'java' | 'bedrock') => void;
+    selected: 'java' | 'bedrock' | null;
-    selected?: 'java' | 'bedrock' | null;
   }
 
-  let { onSelect, selected = null }: Props = $props();
+  let { selected = $bindable() }: Props = $props();
 </script>
 
 <div>
@@ -15,7 +14,7 @@
       type="button"
       class="btn btn-outline h-auto py-4 flex-col gap-1"
       class:btn-active={selected === 'java'}
-      onclick={() => onSelect('java')}
+      onclick={() => (selected = 'java')}
     >
       <span class="text-lg">Java (PC)</span>
       <span class="text-xs font-normal">Anmeldung über die Webseite</span>
@@ -24,7 +23,7 @@
       type="button"
       class="btn btn-outline h-auto py-4 flex-col gap-1"
       class:btn-active={selected === 'bedrock'}
-      onclick={() => onSelect('bedrock')}
+      onclick={() => (selected = 'bedrock')}
     >
       <span class="text-lg">Bedrock (Konsole und Handy)</span>
       <span class="text-xs font-normal">Anmeldung über den Minecraft-Server</span>

src/app/website/signup/SignupFlow.svelte

@@ -22,7 +22,7 @@
 <div>
   <h1 class="text-center text-3xl lg:text-5xl">Anmeldung</h1>
 
-  <EditionSelection onSelect={(edition) => (step = edition)} selected={step} />
+  <EditionSelection bind:selected={step} />
 
   <div class:hidden={step !== 'java'}>
     <JavaSignup