refactor: move shared utilities to lib/shared/ sub-packages

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

frontend/src/lib/shared/utils/requiredFields.test.ts

@@ -0,0 +1,36 @@
+import { describe, it, expect } from 'vitest';
+import { countRequiredFilled } from './requiredFields';
+
+describe('countRequiredFilled', () => {
+	it('returns 0 when all three fields are empty', () => {
+		expect(countRequiredFilled('', '', '')).toBe(0);
+	});
+
+	it('returns 1 when only title is set', () => {
+		expect(countRequiredFilled('Ein Brief', '', '')).toBe(1);
+	});
+
+	it('returns 1 when only dateIso is set', () => {
+		expect(countRequiredFilled('', '1920-05-01', '')).toBe(1);
+	});
+
+	it('returns 1 when only senderId is set', () => {
+		expect(countRequiredFilled('', '', 'person-uuid')).toBe(1);
+	});
+
+	it('returns 2 when title and dateIso are set', () => {
+		expect(countRequiredFilled('Ein Brief', '1920-05-01', '')).toBe(2);
+	});
+
+	it('returns 2 when title and senderId are set', () => {
+		expect(countRequiredFilled('Ein Brief', '', 'person-uuid')).toBe(2);
+	});
+
+	it('returns 2 when dateIso and senderId are set', () => {
+		expect(countRequiredFilled('', '1920-05-01', 'person-uuid')).toBe(2);
+	});
+
+	it('returns 3 when all three fields are set', () => {
+		expect(countRequiredFilled('Ein Brief', '1920-05-01', 'person-uuid')).toBe(3);
+	});
+});