Add: CSV import with Replace All feature and auto-create categories

- Added Replace All checkbox to import UI for clean slate imports
- Implemented DeleteAllProductsAndCategoriesAsync for complete data wipe
- Added auto-creation of categories during CSV import
- Created products_import.csv with 13 products across 4 categories
- Added comprehensive IMPORT_INSTRUCTIONS.md documentation

Technical changes:
- ProductImportService: Added replaceAll parameter to all import methods
- ProductImportService: Categories now auto-created if missing from CSV
- ProductsController: Added replaceAll parameter to Import action
- Import.cshtml: Added Replace All checkbox with danger warnings

Categories: Flour, Cereal, Vitamins, Herbal
Products: 13 products with full variant pricing structures

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

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

LittleShop/Areas/Admin/Views/Products/Import.cshtml

@@ -48,6 +48,18 @@
                         </div>
                     </div>
 
+                    <div class="mb-3">
+                        <div class="form-check">
+                            <input type="checkbox" name="replaceAll" id="replaceAll" class="form-check-input" value="true" />
+                            <label for="replaceAll" class="form-check-label">
+                                <strong class="text-danger">Replace All</strong> - Delete all existing products, variants, and categories before import
+                            </label>
+                        </div>
+                        <div class="form-text text-danger">
+                            <i class="fas fa-exclamation-triangle"></i> <strong>Warning:</strong> This will permanently delete ALL existing product data! This action cannot be undone.
+                        </div>
+                    </div>
+
                     <div class="mb-3">
                         <button type="submit" class="btn btn-primary">
                             <i class="fas fa-upload"></i> Import Products