Add Markdown import feature, fix backslash preview bug

- Add "Import Markdown" button in sidebar Tools panel
- Side-by-side modal with live Markdown→Djot conversion
- Converts: bold, italic, strikethrough, highlight, headers, links, etc.
- Inserts at cursor position with proper blank lines
- Fix REST API preview stripping backslashes (removed unnecessary wp_unslash)

Author: dereuromark

src/Blocks/DjotBlock.php

@@ -86,7 +86,8 @@ public function registerRestRoute(): void
                     // which breaks raw HTML blocks. Security is handled by:
                     // 1. permission_callback requiring edit_posts capability
                     // 2. Profile-based feature restrictions in the converter
-                    'sanitize_callback' => static fn (string $value): string => wp_unslash($value),
+                    // Note: No sanitization needed - apiFetch sends JSON which
+                    // doesn't have WordPress magic quotes issues
                 ],
             ],
         ]);