feat(catalog): full vehicle selector flow in brand catalog

Brand catalog now follows the same navigation as the regular catalog:
1. Brands -> 2. Models -> 3. Years -> 4. Engines -> 5. Categories -> 6. Parts

Backend:
- Add /mye-parts endpoint for MYE-specific parts with category filter
- Uses existing /models, /years, /engines, /categories endpoints

Frontend:
- Complete rewrite of brand-catalog.js with breadcrumb navigation
- State machine: brands -> models -> years -> engines -> categories -> parts
- Search and pagination preserved at parts level
- Breadcrumb allows jumping back to any previous step

pos/templates/catalog.html

@@ -308,6 +308,6 @@
     }
   </script>
   <script src="/pos/static/js/pwa-install.js" defer></script>
-  <script src="/pos/static/js/brand-catalog.js?v=3" defer></script>
+  <script src="/pos/static/js/brand-catalog.js?v=4" defer></script>
 </body>
 </html>