Replace spaces with underscores in CSV column names

Update CSV export column headers to use underscores instead of spaces for better compatibility with data processing tools:
- Vendor Product number → Vendor_product_number
- Vendor code → Vendor_code
- Vendor name → Vendor_name
- Dealer product number → Dealer_product_number
- Zip code → Zip_code

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Model/ReportGenerator.php

@@ -75,18 +75,18 @@ class ReportGenerator
         $sql = "
         SELECT
             DATE_FORMAT(o.created_at, '%m/%d/%Y') AS 'Date',
-            oi.sku AS 'Vendor Product number',
+            oi.sku AS 'Vendor_product_number',
             oi.name AS 'Description',
-            '' AS 'Vendor code',
-            '' AS 'Vendor name',
-            o.increment_id AS 'Dealer product number',
+            '' AS 'Vendor_code',
+            '' AS 'Vendor_name',
+            o.increment_id AS 'Dealer_product_number',
             CASE
                 WHEN sa.country_id = 'US' THEN 'United States'
                 ELSE sa.country_id
             END AS 'Country',
             sa.city AS 'City',
             sa.region AS 'State',
-            sa.postcode AS 'Zip code',
+            sa.postcode AS 'Zip_code',
             CAST(oi.qty_ordered AS UNSIGNED) AS 'Quantity',
             COALESCE(
                 NULLIF(oi.base_cost, 0),