chore: add code block demo to dev app

apps/dev/src/components/Editor.vue

@@ -14,6 +14,13 @@ This is a **block editor** with markdown decorations.
 
 📅 2026-05-20 is a date
 
+\`\`\`python
+def hello():
+    print("Hello from CM6!")
+    for i in range(3):
+        print(f"Line {i}")
+\`\`\`
+
 property:: This is a property line
 another:: [[Page reference]] in a property
 `)