UX: Clarify document/library creation flow

- Replace direct 'New Document' with modal wizard that asks for library first
- Separate 'New Document' (📄) and 'New Library' (📁) buttons in header
- Update sidebar quick links to show both options with clear icons
- Add modal for library creation with name input
- Add CSS for form-control and modal-close button styling
- Minor improvements to document.js (type=button, event.stopPropagation)
- Keyboard shortcut Ctrl+N now opens document creation modal

public/js/app.js

@@ -143,6 +143,6 @@ document.addEventListener('keydown', (e) => {
   }
   if ((e.ctrlKey || e.metaKey) && e.key === 'n' && window.app.state.view === 'dashboard') {
     e.preventDefault();
-    window.app.navigate('editor');
+    window.showNewDocModal();
   }
 });