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>

.gitignore

@@ -14,8 +14,8 @@ dist/
 downloads/
 eggs/
 .eggs/
-lib/
-lib64/
+/lib/
+/lib64/
 parts/
 sdist/
 var/