Implement client-side validation and sanitization

Add UX-focused validation to prevent user accidents and provide immediate feedback. Includes DOMPurify integration for basic content sanitization and real-time form validation.

Features:
- Real-time input validation with visual feedback
- Field-type specific validation (text, URL, markdown)
- DOMPurify integration for display sanitization
- Validation messages with auto-dismiss
- Prevents common user mistakes (HTML in text fields, invalid URLs)
- Client-side sanitization for markdown rendering
- Maintains security awareness while focusing on user experience

Note: This is UX-focused validation - server-side validation remains the primary security boundary.

demo-site/about.html

@@ -110,6 +110,7 @@
 
     <!-- Insertr JavaScript Library -->
     <script src="https://cdn.jsdelivr.net/npm/marked@16.2.1/lib/marked.umd.js"></script>
+    <script src="https://cdn.jsdelivr.net/npm/dompurify@3.0.5/dist/purify.min.js"></script>
     <script src="insertr/insertr.js"></script>
 </body>
 </html>