feat(events): add CSS template selector to event edit page

- Add CSS template selector to ThemeCustomizerEnhanced component
- Rename "Custom CSS" to "Event-specific Custom CSS" for clarity
- Load and save css_template_id when editing events
- Fetch CSS templates when entering edit mode on EventDetailsPage
- Pass CSS template props to ThemeEditorModal
- Add backend validation for css_template_id field

backend/src/routes/adminEvents.js

@@ -581,7 +581,8 @@ router.put('/:id', adminAuth, requirePermission('events.edit'), [
       throw new Error('Password must be at least 6 characters long');
     }
     return true;
-  })
+  }),
+  body('css_template_id').optional({ nullable: true, checkFalsy: true }).isInt()
 ], async (req, res) => {
   try {
     const errors = validationResult(req);