Vollbild-Knopf im Startbildschirm hinzufügen

src/lib/components/shared/FullscreenToggle.svelte

@@ -0,0 +1,72 @@
+<script lang="ts">
+  // Vollbild umschalten via Fullscreen-API. Wird ausgeblendet, wo die API fehlt (z. B. iPhone-Safari).
+  const supported =
+    typeof document !== 'undefined' && typeof document.documentElement.requestFullscreen === 'function';
+
+  let isFullscreen = $state(false);
+
+  function sync() {
+    isFullscreen = !!document.fullscreenElement;
+  }
+
+  $effect(() => {
+    sync();
+    document.addEventListener('fullscreenchange', sync);
+    return () => document.removeEventListener('fullscreenchange', sync);
+  });
+
+  function toggle() {
+    if (document.fullscreenElement) {
+      document.exitFullscreen?.();
+    } else {
+      document.documentElement.requestFullscreen?.().catch(() => {});
+    }
+  }
+</script>
+
+{#if supported}
+  <button
+    class="fs-toggle"
+    type="button"
+    aria-label={isFullscreen ? 'Vollbild beenden' : 'Vollbild'}
+    onclick={toggle}
+  >
+    {#if isFullscreen}
+      <svg width="30" height="30" viewBox="0 0 32 32" aria-hidden="true">
+        <path
+          d="M13 4 L13 13 L4 13 M19 4 L19 13 L28 13 M13 28 L13 19 L4 19 M19 28 L19 19 L28 19"
+          fill="none"
+          stroke="currentColor"
+          stroke-width="2.6"
+          stroke-linecap="round"
+          stroke-linejoin="round"
+        />
+      </svg>
+    {:else}
+      <svg width="30" height="30" viewBox="0 0 32 32" aria-hidden="true">
+        <path
+          d="M4 11 L4 4 L11 4 M28 11 L28 4 L21 4 M4 21 L4 28 L11 28 M28 21 L28 28 L21 28"
+          fill="none"
+          stroke="currentColor"
+          stroke-width="2.6"
+          stroke-linecap="round"
+          stroke-linejoin="round"
+        />
+      </svg>
+    {/if}
+  </button>
+{/if}
+
+<style>
+  .fs-toggle {
+    width: 64px;
+    height: 64px;
+    border-radius: 50%;
+    background: rgba(255, 255, 255, 0.18);
+    color: var(--c-text-on-dark);
+    display: grid;
+    place-items: center;
+    transition: transform 0.12s ease;
+  }
+  .fs-toggle:active { transform: scale(0.92); }
+</style>

src/lib/screens/HomeScreen.svelte

@@ -6,6 +6,7 @@
   import { play, unlockAudio } from '../audio/soundManager';
   import NumberCard from '../components/home/NumberCard.svelte';
   import SoundToggle from '../components/shared/SoundToggle.svelte';
+  import FullscreenToggle from '../components/shared/FullscreenToggle.svelte';
   import IconButton from '../components/shared/IconButton.svelte';
 
   function handlePick(t: Target) {
@@ -20,6 +21,8 @@
   <header class="topbar">
     <SoundToggle />
     <h1 class="visually-hidden">Zahlzerlegung</h1>
+    <div class="actions">
+      <FullscreenToggle />
       <IconButton label="Einstellungen" onClick={goSettings}>
         <svg width="34" height="34" viewBox="0 0 32 32" aria-hidden="true">
           <path
@@ -29,6 +32,7 @@
           />
         </svg>
       </IconButton>
+    </div>
   </header>
 
   <div class="cards-grid">
@@ -51,6 +55,11 @@
     justify-content: space-between;
     align-items: center;
   }
+  .actions {
+    display: flex;
+    gap: 10px;
+    align-items: center;
+  }
   .visually-hidden {
     position: absolute;
     width: 1px;