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/admin/login.html

@@ -117,8 +117,22 @@
     <!-- 4. API Client -->
     <script src="{{ url_for('static', path='shared/js/api-client.js') }}"></script>
 
-    <!-- 5. Alpine.js v3 -->
-    <script defer src="https://cdn.jsdelivr.net/npm/alpinejs@3.14.0/dist/cdn.min.js"></script>
+    <!-- 5. Alpine.js v3 with CDN fallback -->
+    <script>
+    (function() {
+        var script = document.createElement('script');
+        script.defer = true;
+        script.src = 'https://cdn.jsdelivr.net/npm/alpinejs@3.14.0/dist/cdn.min.js';
+        script.onerror = function() {
+            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/lib/alpine.min.js") }}';
+            document.head.appendChild(fallbackScript);
+        };
+        document.head.appendChild(script);
+    })();
+    </script>
 
     <!-- 6. Login Logic -->
     <script src="{{ url_for('static', path='admin/js/login.js') }}"></script>