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/css/style.css

@@ -932,6 +932,41 @@ ul, ol {
   border-top: 1px solid var(--color-border);
 }
 
+.modal-close {
+  background: none;
+  border: none;
+  color: var(--color-text-muted);
+  cursor: pointer;
+  padding: var(--space-1);
+  border-radius: var(--radius-sm);
+  font-size: 1rem;
+}
+
+.modal-close:hover {
+  background: var(--color-hover);
+  color: var(--color-text);
+}
+
+.form-control {
+  width: 100%;
+  padding: var(--space-3);
+  background: var(--color-bg);
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-md);
+  color: var(--color-text);
+  font-size: 0.875rem;
+}
+
+.form-control:focus {
+  outline: none;
+  border-color: var(--color-accent);
+}
+
+.form-control option {
+  background: var(--color-surface);
+  color: var(--color-text);
+}
+
 /* === Login Screen === */
 .login-screen {
   min-height: 100vh;