feat(app): LanguageSwitcher-komponente mit de/en-buttons

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src/lib/components/LanguageSwitcher.svelte

@@ -0,0 +1,48 @@
+<script lang="ts">
+  import { t, activeLocale, SUPPORTED_LOCALES } from '$lib/i18n';
+  import type { SupportedLocale } from '$lib/i18n/activeLocale';
+
+  let current = $state<SupportedLocale>('de');
+  activeLocale.subscribe((v) => (current = v));
+
+  function select(lang: SupportedLocale) {
+    activeLocale.set(lang);
+  }
+</script>
+
+<div class="switcher" role="group" aria-label={$t('lang.switch_aria')}>
+  {#each SUPPORTED_LOCALES as code}
+    <button
+      type="button"
+      class="btn"
+      class:active={current === code}
+      aria-pressed={current === code}
+      onclick={() => select(code)}
+    >{code.toUpperCase()}</button>
+  {/each}
+</div>
+
+<style>
+  .switcher {
+    display: inline-flex;
+    gap: 0.25rem;
+    margin-left: 0.5rem;
+  }
+  .btn {
+    background: transparent;
+    border: 1px solid var(--border);
+    color: var(--muted);
+    border-radius: 3px;
+    padding: 1px 7px;
+    font-size: 0.8rem;
+    cursor: pointer;
+    font-family: inherit;
+  }
+  .btn:hover {
+    color: var(--fg);
+  }
+  .btn.active {
+    color: var(--accent);
+    border-color: var(--accent);
+  }
+</style>