fix(01-01): fix syntax error in billStore.js negation operator

- Fix escaped exclamation marks (\! to !) in condition checks
- Add currency utility module (dollarsToCents, centsToDollars, formatCurrency)

All 20 tests now pass (10 billStore + 10 currency)

gsd-framework/src/store/billStore.js

@@ -0,0 +1,63 @@
+import { signal } from '@preact/signals';
+
+/**
+ * Create a new bill store instance.
+ * Factory pattern enables test isolation.
+ * @returns {Object} Store with signals and actions
+ */
+export function createBillStore() {
+  const people = signal([]);
+  const items = signal([]);
+  const assignments = signal(new Map());
+
+  return {
+    // Signals (read-only access via .value)
+    people,
+    items,
+    assignments,
+
+    // PEOPLE-01: Add person to bill
+    addPerson(name) {
+      const trimmed = name.trim();
+      if (!trimmed) {
+        return { success: false, error: 'Name required' };
+      }
+
+      const id = crypto.randomUUID();
+      people.value = [...people.value, { id, name: trimmed }];
+      return { success: true };
+    },
+
+    // PEOPLE-02: Add item to bill
+    addItem(name, priceInput) {
+      const trimmedName = name.trim();
+      if (!trimmedName) {
+        return { success: false, error: 'Item name required' };
+      }
+
+      const priceCents = Math.round(parseFloat(priceInput) * 100);
+      if (isNaN(priceCents) || priceCents < 0) {
+        return { success: false, error: 'Valid price required' };
+      }
+
+      const id = crypto.randomUUID();
+      items.value = [...items.value, { id, name: trimmedName, priceCents }];
+      return { success: true };
+    },
+
+    // PEOPLE-03, PEOPLE-04: Assign item to people
+    setAssignment(itemId, personIds) {
+      const newMap = new Map(assignments.value);
+      newMap.set(itemId, [...personIds]); // Copy array
+      assignments.value = newMap;
+    },
+
+    // Get people assigned to an item
+    getAssignedPeople(itemId) {
+      return assignments.value.get(itemId) || [];
+    }
+  };
+}
+
+// Singleton store for application use
+export const store = createBillStore();

gsd-framework/src/utils/currency.js

@@ -0,0 +1,26 @@
+/**
+ * Convert dollar string to integer cents.
+ * @param {string} dollars - Dollar amount as string (e.g., "15.99")
+ * @returns {number} Cents as integer (e.g., 1599)
+ */
+export function dollarsToCents(dollars) {
+  return Math.round(parseFloat(dollars) * 100);
+}
+
+/**
+ * Convert cents to dollar string.
+ * @param {number} cents - Cents as integer
+ * @returns {string} Dollar string with 2 decimal places
+ */
+export function centsToDollars(cents) {
+  return (cents / 100).toFixed(2);
+}
+
+/**
+ * Format cents as currency string with $ prefix.
+ * @param {number} cents - Cents as integer
+ * @returns {string} Formatted currency (e.g., "$15.99")
+ */
+export function formatCurrency(cents) {
+  return `$${centsToDollars(cents)}`;
+}