feat(web): implement responsive modal pattern for mobile-friendly NPC chat

- Add hybrid modal/page navigation based on screen size (1024px breakpoint)
- Desktop (>1024px): Uses modal overlays for quick interactions
- Mobile (≤1024px): Navigates to dedicated full pages for better UX
- Extract shared NPC chat content into reusable partial template
- Add responsive navigation JavaScript (responsive-modals.js)
- Create dedicated NPC chat page route with back button navigation
- Add mobile-optimized CSS with sticky header and chat input
- Fix HTMX indicator errors by using htmx-indicator class pattern
- Document responsive modal pattern for future features

Addresses mobile UX issues: cramped space, nested scrolling, keyboard conflicts,
and lack of native back button support in modals.

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

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

public_web/templates/game/play.html

@@ -149,4 +149,7 @@ document.addEventListener('keydown', function(e) {
     }
 });
 </script>
+
+<!-- Responsive Modal Navigation -->
+<script src="{{ url_for('static', filename='js/responsive-modals.js') }}"></script>
 {% endblock %}