FIX: CRITICAL use nullish coalescing for product quantity in useMarket composable

- Updated the quantity assignment in the useMarket composable to use nullish coalescing (??) instead of logical OR (||). This change ensures that a quantity of 0 is preserved, preventing unintended defaults when productData.quantity is explicitly set to 0.

These changes improve the accuracy of product quantity handling in the market module.

CLAUDE.md

@@ -730,6 +730,33 @@ export function useMyModule() {
 - **ALWAYS extend BaseService for module services**
 - **NEVER create direct dependencies between modules**
 
+### **⚠️ CRITICAL: JavaScript Falsy Value Bug Prevention**
+
+**ALWAYS use nullish coalescing (`??`) instead of logical OR (`||`) for numeric defaults:**
+
+```typescript
+// ❌ WRONG: Treats 0 as falsy, defaults to 1 even when quantity is validly 0
+quantity: productData.quantity || 1
+
+// ✅ CORRECT: Only defaults to 1 when quantity is null or undefined
+quantity: productData.quantity ?? 1
+```
+
+**Why this matters:**
+- JavaScript falsy values include: `false`, `0`, `""`, `null`, `undefined`, `NaN`
+- Using `||` for defaults will incorrectly override valid `0` values
+- This caused a critical bug where products with quantity `0` displayed as quantity `1`
+- The `??` operator only triggers for `null` and `undefined`, preserving valid `0` values
+
+**Common scenarios where this bug occurs:**
+- Product quantities, prices, counters (any numeric value where 0 is valid)
+- Boolean flags where `false` is a valid state
+- Empty strings that should be preserved vs. undefined strings
+
+**Rule of thumb:**
+- Use `||` only when `0`, `false`, or `""` should trigger the default
+- Use `??` when only `null`/`undefined` should trigger the default (most cases)
+
 **Build Configuration:**
 - Vite config includes PWA, image optimization, and bundle analysis
 - Manual chunking strategy for vendor libraries (vue-vendor, ui-vendor, shadcn)

src/modules/market/composables/useMarket.ts

@@ -310,7 +310,7 @@ export function useMarket() {
             description: productData.description || '',
             price: productData.price || 0,
             currency: productData.currency || 'sats',
-            quantity: productData.quantity || 1,
+            quantity: productData.quantity ?? 1, // Use nullish coalescing to preserve 0
             images: productData.images || [],
             categories: categories,
             createdAt: latestEvent.created_at,
@@ -502,7 +502,7 @@ export function useMarket() {
           description: productData.description || '',
           price: productData.price || 0,
           currency: productData.currency || 'sats',
-          quantity: productData.quantity || 1,
+          quantity: productData.quantity ?? 1, // Use nullish coalescing to preserve 0
           images: productData.images || [],
           categories: categories,
           createdAt: event.created_at,