feat(01-01): set up test infrastructure with Vitest

- Add package.json with Vitest, jsdom, Vite, and Preact dependencies
- Configure Vitest with jsdom environment for component testing
- Create test stubs for billStore (PEOPLE-01, PEOPLE-02, PEOPLE-03, PEOPLE-04)
- Create test stubs for currency utilities (dollarsToCents, centsToDollars, formatCurrency)

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

tests/billStore.test.js

@@ -0,0 +1,83 @@
+import { describe, it, expect, beforeEach } from 'vitest';
+import { createBillStore } from '../src/store/billStore.js';
+
+describe('BillStore', () => {
+  let store;
+
+  beforeEach(() => {
+    store = createBillStore();
+  });
+
+  describe('addPerson (PEOPLE-01)', () => {
+    it('adds person with valid name', () => {
+      const result = store.addPerson('Alice');
+      expect(result.success).toBe(true);
+      expect(store.people.value).toHaveLength(1);
+      expect(store.people.value[0].name).toBe('Alice');
+    });
+
+    it('rejects empty name', () => {
+      const result = store.addPerson('');
+      expect(result.success).toBe(false);
+    });
+
+    it('trims whitespace from name', () => {
+      const result = store.addPerson('  Bob  ');
+      expect(result.success).toBe(true);
+      expect(store.people.value[0].name).toBe('Bob');
+    });
+  });
+
+  describe('addItem (PEOPLE-02)', () => {
+    it('adds item with name and price', () => {
+      const result = store.addItem('Pizza', '15.99');
+      expect(result.success).toBe(true);
+      expect(store.items.value[0].priceCents).toBe(1599);
+    });
+
+    it('rejects empty item name', () => {
+      const result = store.addItem('', '10.00');
+      expect(result.success).toBe(false);
+    });
+
+    it('rejects negative price', () => {
+      const result = store.addItem('Item', '-5.00');
+      expect(result.success).toBe(false);
+    });
+
+    it('rejects non-numeric price', () => {
+      const result = store.addItem('Item', 'abc');
+      expect(result.success).toBe(false);
+    });
+  });
+
+  describe('setAssignment (PEOPLE-03, PEOPLE-04)', () => {
+    it('assigns item to one person', () => {
+      store.addPerson('Alice');
+      const personId = store.people.value[0].id;
+      store.addItem('Salad', '10.00');
+      const itemId = store.items.value[0].id;
+
+      store.setAssignment(itemId, [personId]);
+      expect(store.getAssignedPeople(itemId)).toEqual([personId]);
+    });
+
+    it('assigns item to multiple people (shared)', () => {
+      store.addPerson('Alice');
+      store.addPerson('Bob');
+      const [id1, id2] = store.people.value.map(p => p.id);
+      store.addItem('Nachos', '12.00');
+      const itemId = store.items.value[0].id;
+
+      store.setAssignment(itemId, [id1, id2]);
+      expect(store.getAssignedPeople(itemId)).toHaveLength(2);
+    });
+
+    it('returns empty array for unassigned item', () => {
+      store.addItem('Standalone', '5.00');
+      const itemId = store.items.value[0].id;
+
+      expect(store.getAssignedPeople(itemId)).toEqual([]);
+    });
+  });
+});

tests/currency.test.js

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+import { describe, it, expect } from 'vitest';
+import { dollarsToCents, centsToDollars, formatCurrency } from '../src/utils/currency.js';
+
+describe('currency utilities', () => {
+  describe('dollarsToCents', () => {
+    it('converts decimal string to cents', () => {
+      expect(dollarsToCents('15.99')).toBe(1599);
+    });
+
+    it('converts integer string to cents', () => {
+      expect(dollarsToCents('20')).toBe(2000);
+    });
+
+    it('handles single decimal place', () => {
+      expect(dollarsToCents('10.5')).toBe(1050);
+    });
+
+    it('handles zero', () => {
+      expect(dollarsToCents('0')).toBe(0);
+    });
+
+    it('rounds floating point precision', () => {
+      // 10.99 * 100 = 1098.9999... in JS
+      expect(dollarsToCents('10.99')).toBe(1099);
+    });
+  });
+
+  describe('centsToDollars', () => {
+    it('converts cents to dollar string', () => {
+      expect(centsToDollars(1599)).toBe('15.99');
+    });
+
+    it('handles zero', () => {
+      expect(centsToDollars(0)).toBe('0.00');
+    });
+
+    it('pads single digit cents', () => {
+      expect(centsToDollars(1050)).toBe('10.50');
+    });
+  });
+
+  describe('formatCurrency', () => {
+    it('formats cents as dollar string with $ prefix', () => {
+      expect(formatCurrency(1599)).toBe('$15.99');
+    });
+
+    it('formats zero', () => {
+      expect(formatCurrency(0)).toBe('$0.00');
+    });
+  });
+});