[ADD] product_main_seller: Restore OCA addon from original version

- Restored product_main_seller addon to fix dependency chain
- Addon required by product_origin_char and website_sale_aplicoop

[IMP] website_sale_aplicoop: Add demo data configuration to manifest

- Updated __manifest__.py to include demo data files
- Demo data includes partners, suppliers, products, group orders, and sale orders
- Demo data successfully loads during module installation

product_main_seller/README.md

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+# Product Main Seller
+
+## Summary
+
+Adds a "Main Vendor" field to products that automatically tracks the primary supplier based on supplierinfo sequence.
+
+## Features
+
+- **Automatic Main Vendor**: Computed field showing the primary supplier
+- **Based on Sequence**: Uses the supplierinfo with the lowest sequence number
+- **Visible in Views**: Shows in product form and tree views
+- **Searchable**: Can search and filter products by main vendor
+- **Related Fields**: Access to vendor name and reference
+
+## Technical Details
+
+### Models Extended
+
+- `product.template`: Main product template
+- `product.product`: Product variants
+
+### Fields Added
+
+- `main_seller_id` (Many2one → res.partner, computed, stored)
+  - The main vendor for this product
+  - Computed from `seller_ids` with lowest sequence
+  - Stored for performance
+  - Searchable and filterable
+
+### Computation Logic
+
+The main vendor is determined by:
+1. Looking at all supplierinfo records (`seller_ids`)
+2. Filtering for valid suppliers (active partners)
+3. Selecting the one with the **lowest sequence** number
+4. If no suppliers, returns empty
+
+## Dependencies
+
+- `purchase` (Odoo core)
+
+## Installation
+
+```bash
+docker-compose exec odoo odoo -d odoo -u product_main_seller --stop-after-init
+```
+
+## Usage
+
+## Usage
+
+### Viewing Main Vendor
+
+1. Open a product form (Products > Products > [Product])
+2. See "Main Vendor" field (usually in Purchase tab)
+3. Field is automatically computed from vendor list
+
+### Changing Main Vendor
+
+To change the main vendor:
+1. Go to product form > Purchase tab
+2. Edit the vendor list (`seller_ids`)
+3. Change the sequence numbers (lower = more priority)
+4. Save - the "Main Vendor" will update automatically
+
+### Searching by Vendor
+
+```python
+# Find all products from a specific vendor
+products = self.env['product.template'].search([
+    ('main_seller_id', '=', vendor_id)
+])
+```
+
+## Use Cases in Kidekoop
+
+This module is critical for:
+- Vendor performance analysis
+- Purchase order management
+- Inventory planning by supplier
+- Default supplier selection in purchase workflows
+
+## Views Modified
+
+- Product Template Form View
+- Product Template Tree View
+- Product Variant Form View
+
+## Translations
+
+- ✅ Spanish (es)
+- ✅ Euskera (eu)
+
+Located in `i18n/` directory.
+
+---
+
+**Version**: 18.0.1.0.0
+**Category**: Purchase
+**License**: AGPL-3