Fix invalid unit_of_measure 'dozen' causing 422 API errors

**Issue:**
Creating ingredients failed with 422 error:
```
Input should be 'kg', 'g', 'l', 'ml', 'units', 'pcs', 'pkg', 'bags' or 'boxes'
input: "dozen"
```

**Root Cause:**
Frontend was using units not supported by backend UnitOfMeasure enum:
- "dozen" (docena)
- "cup" (taza)
- "tbsp" (cucharada)
- "tsp" (cucharadita)
- "piece" → should be "pcs"
- "package" → should be "pkg"
- "bag" → should be "bags"
- "box" → should be "boxes"

**Backend Supported Units (inventory.ts:28-38):**
kg, g, l, ml, units, pcs, pkg, bags, boxes

**Solution:**
Replaced all invalid units with backend-compatible ones across codebase.

**Files Modified:**

1. **UploadSalesDataStep.tsx:604**
   - Before: ['kg', 'g', 'L', 'ml', 'units', 'dozen']
   - After: ['kg', 'g', 'l', 'ml', 'units', 'pcs', 'pkg', 'bags', 'boxes']

2. **BatchAddIngredientsModal.tsx:53**
   - Before: ['kg', 'g', 'L', 'ml', 'units', 'dozen']
   - After: ['kg', 'g', 'l', 'ml', 'units', 'pcs', 'pkg', 'bags', 'boxes']

3. **QuickAddIngredientModal.tsx:69**
   - Before: ['kg', 'g', 'L', 'ml', 'units', 'dozen']
   - After: ['kg', 'g', 'l', 'ml', 'units', 'pcs', 'pkg', 'bags', 'boxes']

4. **inventory/index.ts:51-62**
   - Removed: 'piece', 'package', 'bag', 'box', 'dozen', 'cup', 'tbsp', 'tsp'
   - Added: 'units', 'pcs', 'pkg', 'bags', 'boxes'
   - Added comment: "must match backend UnitOfMeasure enum exactly"

5. **ingredientHelpers.ts:168**
   - Eggs unit changed from 'dozen' → 'units'

6. **utils/constants.ts:77-87**
   - Removed volume units: cup, tbsp, tsp
   - Removed count units: piece, dozen, package, bag, box
   - Added: units, pcs, pkg, bags, boxes
   - Now matches backend enum exactly

**Also Fixed:**
- Changed 'L' to lowercase 'l' for consistency with backend

**Impact:**
✅ All ingredient creation now uses valid backend units
✅ No more 422 validation errors
✅ Frontend/backend unit enums synchronized

**Build Status:** ✓ Successful in 22.23s

frontend/src/utils/constants.ts

@@ -77,16 +77,13 @@ export const UNITS_OF_MEASURE = {
   volume: {
     l: { label: 'Litro', symbol: 'l', factor: 1000 },
     ml: { label: 'Mililitro', symbol: 'ml', factor: 1 },
-    cup: { label: 'Taza', symbol: 'taza', factor: 240 },
-    tbsp: { label: 'Cucharada', symbol: 'cda', factor: 15 },
-    tsp: { label: 'Cucharadita', symbol: 'cdta', factor: 5 },
   },
   count: {
-    piece: { label: 'Pieza', symbol: 'pz', factor: 1 },
-    dozen: { label: 'Docena', symbol: 'doc', factor: 12 },
-    package: { label: 'Paquete', symbol: 'paq', factor: 1 },
-    bag: { label: 'Bolsa', symbol: 'bolsa', factor: 1 },
-    box: { label: 'Caja', symbol: 'caja', factor: 1 },
+    units: { label: 'Unidades', symbol: 'ud', factor: 1 },
+    pcs: { label: 'Piezas', symbol: 'pz', factor: 1 },
+    pkg: { label: 'Paquetes', symbol: 'paq', factor: 1 },
+    bags: { label: 'Bolsas', symbol: 'bolsa', factor: 1 },
+    boxes: { label: 'Cajas', symbol: 'caja', factor: 1 },
   },
 } as const;