Fix FullCalendar loading error on PTO page

Resolves "Uncaught TypeError: $(...).fullCalendar is not a function"
by loading FullCalendar and Moment.js libraries from CDN.

Changes:
- Add Moment.js 2.29.4 from CDN to application layout
- Add FullCalendar 3.10.5 CSS and JS from CDN
- Remove local javascript_include_tag calls from PTO page
- Ensure libraries load before page attempts to initialize calendar

The PTO calendar now loads reliably across page navigations.

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

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

app/views/paid_time_off/index.html.erb

@@ -179,9 +179,6 @@
   </div>
 </div>
 
-<%= javascript_include_tag "moment.min.js" %>
-<%= javascript_include_tag "fullcalendar.min.js" %>
-
 <script type="text/javascript">
 function makeActive() {
   $('li[id="pto"]').addClass('active');