refactor: rename third-party JS libs folder from vendor to lib

Rename static/shared/js/vendor/ to static/shared/js/lib/ to avoid
confusion with the app's "vendor" (seller) dashboard code in
static/vendor/js/.

- Rename directory: vendor/ → lib/
- Update all template references to use new path
- Update CDN fallback documentation
- Fix .gitignore to use /lib/ (root only) instead of lib/ (everywhere)

Third-party libraries:
- alpine.min.js
- chart.umd.min.js
- flatpickr.min.js
- quill.js
- tom-select.complete.min.js

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/shop/base.html

@@ -340,7 +340,7 @@
             console.warn('Alpine.js CDN failed, loading local copy...');
             var fallbackScript = document.createElement('script');
             fallbackScript.defer = true;
-            fallbackScript.src = '{{ url_for("static", path="shared/js/vendor/alpine.min.js") }}';
+            fallbackScript.src = '{{ url_for("static", path="shared/js/lib/alpine.min.js") }}';
             document.head.appendChild(fallbackScript);
         };