fix(i18n): use getLocale() for date formatting in panel header

Replaces hardcoded 'de-DE' with the active Paraglide locale so
dates render in the user's language.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/TranscriptionPanelHeader.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
+import { getLocale } from '$lib/paraglide/runtime.js';
 
 type Props = {
 	mode: 'read' | 'edit';
@@ -14,7 +15,7 @@ let { mode, hasBlocks, blockCount, lastEditedAt, onModeChange, onClose }: Props
 
 const formattedDate = $derived(
 	lastEditedAt
-		? new Intl.DateTimeFormat('de-DE', {
+		? new Intl.DateTimeFormat(getLocale(), {
 				day: 'numeric',
 				month: 'short',
 				year: 'numeric'