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/Controllers/ProductsController.cs

@@ -199,7 +199,7 @@ public class ProductsController : Controller
 
     [HttpPost]
     [ValidateAntiForgeryToken]
-    public async Task<IActionResult> Import(IFormFile file)
+    public async Task<IActionResult> Import(IFormFile file, bool replaceAll = false)
     {
         if (file == null || file.Length == 0)
         {
@@ -216,7 +216,7 @@ public class ProductsController : Controller
         try
         {
             using var stream = file.OpenReadStream();
-            var result = await _importService.ImportFromCsvAsync(stream);
+            var result = await _importService.ImportFromCsvAsync(stream, replaceAll);
 
             ViewData["ImportResult"] = result;
             return View("ImportResult", result);