Add slugify utility for filename sanitization

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

frontend/src/utils/slugify.ts

@@ -0,0 +1,28 @@
+const UMLAUT_MAP: Record<string, string> = {
+  ä: 'ae',
+  ö: 'oe',
+  ü: 'ue',
+  ß: 'ss',
+  Ä: 'Ae',
+  Ö: 'Oe',
+  Ü: 'Ue',
+}
+
+export function slugify(input: string): string {
+  return (
+    input
+      // Transliterate German umlauts
+      .replace(/[äöüßÄÖÜ]/g, (ch) => UMLAUT_MAP[ch] ?? ch)
+      .toLowerCase()
+      // Remove non-ASCII characters
+      .replace(/[^\x20-\x7E]/g, '')
+      // Replace non-alphanumeric characters with hyphens
+      .replace(/[^a-z0-9]+/g, '-')
+      // Collapse consecutive hyphens
+      .replace(/-{2,}/g, '-')
+      // Trim leading/trailing hyphens
+      .replace(/^-|-$/g, '')
+      // Truncate to 60 characters
+      .slice(0, 60)
+  )
+}