feat(shopping): unitFamily-Utility fuer Konsolidierung

Neue Hilfsfunktion `unitFamily` normalisiert Einheiten auf eine
Familien-Kennung ('weight', 'volume' oder lowercase-trim). Wird
in nachfolgenden Konsolidierungs-Schritten der Einkaufsliste
verwendet. Abgedeckt durch 5 Vitest-Unit-Tests (TDD).

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

tests/unit/unit-consolidation.test.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { unitFamily } from '../../src/lib/server/unit-consolidation';
+
+describe('unitFamily', () => {
+  it('maps g and kg to weight', () => {
+    expect(unitFamily('g')).toBe('weight');
+    expect(unitFamily('kg')).toBe('weight');
+  });
+  it('maps ml and l to volume', () => {
+    expect(unitFamily('ml')).toBe('volume');
+    expect(unitFamily('l')).toBe('volume');
+  });
+  it('lowercases and trims unknown units', () => {
+    expect(unitFamily(' Bund ')).toBe('bund');
+    expect(unitFamily('TL')).toBe('tl');
+    expect(unitFamily('Stück')).toBe('stück');
+  });
+  it('is case-insensitive for weight/volume', () => {
+    expect(unitFamily('Kg')).toBe('weight');
+    expect(unitFamily('ML')).toBe('volume');
+  });
+  it('returns empty string for null/undefined/empty', () => {
+    expect(unitFamily(null)).toBe('');
+    expect(unitFamily(undefined)).toBe('');
+    expect(unitFamily('')).toBe('');
+    expect(unitFamily('   ')).toBe('');
+  });
+});