feat: Add allocation method type ONCE (#13700)

### What
Add a new `once` allocation strategy to promotions that limits application to a maximum number of items across the entire cart, rather than per line item.

### Why
Merchants want to create promotions that apply to a limited number of items across the entire cart. For example:
- "Get $10 off, applied to one item only"
- "20% off up to 2 items in your cart"

Current allocation strategies:
- `each`: Applies to each line item independently (respects `max_quantity` per item)
- `across`: Distributes proportionally across all items

Neither supports limiting total applications across the entire cart.

### How

Add `once` to the `ApplicationMethodAllocation` enum.

Behavior:
- Applies promotion to maximum `max_quantity` items across entire cart
- Always prioritizes lowest-priced eligible items first
- Distributes sequentially across items until quota exhausted
- Requires `max_quantity` field to be set

### Example Usage

**Scenario 1: Fixed discount**
```javascript
{
  type: "fixed",
  allocation: "once",
  value: 10,        // $10 off
  max_quantity: 2   // Apply to 2 items max across cart
}

Cart:
- Item A: 3 units @ $100/unit
- Item B: 5 units @ $50/unit (lowest price)

Result: $20 discount on Item B (2 units × $10)
```

**Scenario 2: Distribution across items**
```javascript
{
  type: "fixed",
  allocation: "once",
  value: 5,
  max_quantity: 4
}

Cart:
- Item A: 2 units @ $50/unit
- Item B: 3 units @ $60/unit

Result:
- Item A: $10 discount (2 units × $5)
- Item B: $10 discount (2 units × $5, remaining quota)
```

**Scenario 3: Percentage discount - single item**
```javascript
{
  type: "percentage",
  allocation: "once",
  value: 20,         // 20% off
  max_quantity: 3    // Apply to 3 items max
}

Cart:
- Item A: 5 units @ $100/unit
- Item B: 4 units @ $50/unit (lowest price)

Result: $30 discount on Item B (3 units × $50 × 20% = $30)
```

**Scenario 4: Percentage discount - distributed across items**
```javascript
{
  type: "percentage",
  allocation: "once",
  value: 15,         // 15% off
  max_quantity: 5
}

Cart:
- Item A: 2 units @ $40/unit (lowest price)
- Item B: 4 units @ $80/unit

Result:
- Item A: $12 discount (2 units × $40 × 15% = $12)
- Item B: $36 discount (3 units × $80 × 15% = $36, remaining quota)
Total: $48 discount
```

**Scenario 5: Percentage with max_quantity = 1**
```javascript
{
  type: "percentage",
  allocation: "once",
  value: 25,         // 25% off
  max_quantity: 1    // Only one item
}

Cart:
- Item A: 3 units @ $60/unit
- Item B: 2 units @ $30/unit (lowest price)

Result: $7.50 discount on Item B (1 unit × $30 × 25%)
```

packages/admin/dashboard/src/i18n/translations/$schema.json

@@ -7569,6 +7569,9 @@
             "allocation": {
               "type": "string"
             },
+            "allocationTooltip": {
+              "type": "string"
+            },
             "addCondition": {
               "type": "string"
             },
@@ -8048,9 +8051,22 @@
                   },
                   "required": ["title", "description"],
                   "additionalProperties": false
+                },
+                "once": {
+                  "type": "object",
+                  "properties": {
+                    "title": {
+                      "type": "string"
+                    },
+                    "description": {
+                      "type": "string"
+                    }
+                  },
+                  "required": ["title", "description"],
+                  "additionalProperties": false
                 }
               },
-              "required": ["each", "across"],
+              "required": ["each", "across", "once"],
               "additionalProperties": false
             },
             "code": {

packages/admin/dashboard/src/i18n/translations/en.json

@@ -2021,6 +2021,7 @@
       "campaign": "Campaign",
       "method": "Method",
       "allocation": "Allocation",
+      "allocationTooltip": "Each enforces the quantity limit per item, while Once enforces the quantity limit across the entire cart",
       "addCondition": "Add condition",
       "clearAll": "Clear all",
       "taxInclusive": "Tax Inclusive",
@@ -2162,6 +2163,10 @@
         "across": {
           "title": "Across",
           "description": "Applies value across items"
+        },
+        "once": {
+          "title": "Once",
+          "description": "Applies value to a limited number of items"
         }
       },
       "code": {