fix: Resolve production issues in VPS environment

- Fix toggleCategory JavaScript error by adding null checks for productsGrid
- Fix product import persistence by changing from INSERT OR REPLACE to DELETE + INSERT
- Add automatic default product import on server startup when database is empty
- Ensure product data survives server restarts and imports

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

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

app.js

@@ -2300,10 +2300,10 @@ function toggleCategory(event) {
     categorySection.classList.toggle('collapsed');
     
     if (categorySection.classList.contains('collapsed')) {
-        productsGrid.style.display = 'none';
+        if (productsGrid) productsGrid.style.display = 'none';
         toggle.textContent = '▶';
     } else {
-        productsGrid.style.display = 'block';
+        if (productsGrid) productsGrid.style.display = 'block';
         toggle.textContent = '▼';
     }
 }