Add slugify utility for filename sanitization

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

frontend/src/utils/__tests__/slugify.spec.ts

@@ -0,0 +1,69 @@
+import { describe, it, expect } from 'vitest'
+import { slugify } from '../slugify'
+
+describe('slugify', () => {
+  it('converts to lowercase', () => {
+    expect(slugify('Hello World')).toBe('hello-world')
+  })
+
+  it('replaces spaces with hyphens', () => {
+    expect(slugify('foo bar baz')).toBe('foo-bar-baz')
+  })
+
+  it('transliterates German umlauts', () => {
+    expect(slugify('Ärger über Öl füßen')).toBe('aerger-ueber-oel-fuessen')
+  })
+
+  it('transliterates uppercase umlauts', () => {
+    expect(slugify('Ä Ö Ü')).toBe('ae-oe-ue')
+  })
+
+  it('transliterates ß', () => {
+    expect(slugify('Straße')).toBe('strasse')
+  })
+
+  it('removes non-ASCII characters after transliteration', () => {
+    expect(slugify('Café résumé')).toBe('caf-rsum')
+  })
+
+  it('replaces special characters with hyphens', () => {
+    expect(slugify('hello@world! #test')).toBe('hello-world-test')
+  })
+
+  it('collapses consecutive hyphens', () => {
+    expect(slugify('foo---bar')).toBe('foo-bar')
+  })
+
+  it('trims leading and trailing hyphens', () => {
+    expect(slugify('--hello--')).toBe('hello')
+  })
+
+  it('truncates to 60 characters', () => {
+    const long = 'a'.repeat(80)
+    expect(slugify(long).length).toBeLessThanOrEqual(60)
+  })
+
+  it('does not break mid-word when truncating', () => {
+    // 60 chars of 'a' should just be 60 a's (no word boundary issue)
+    const result = slugify('a'.repeat(65))
+    expect(result.length).toBe(60)
+    expect(result).toBe('a'.repeat(60))
+  })
+
+  it('handles empty string', () => {
+    expect(slugify('')).toBe('')
+  })
+
+  it('handles string that becomes empty after processing', () => {
+    expect(slugify('!@#$%')).toBe('')
+  })
+
+  it('handles emoji', () => {
+    const result = slugify('Party 🎉 time')
+    expect(result).toBe('party-time')
+  })
+
+  it('produces Sommerfest am See example from spec', () => {
+    expect(slugify('Sommerfest am See')).toBe('sommerfest-am-see')
+  })
+})