fix: send button clearance and code block display mode

- Increase editable min-height from 40px to 52px and padding from 8px
  to 10px so the send button (32px + 8px bottom offset) has 12px
  clearance from the toolbar border instead of 0px
- Add display: block to .mc-code-block — Lexical CodeNode renders as
  <code> which is inline by default, causing border/padding to split
  visually across lines instead of forming a proper block
- Revert empty code block to code.select() without synthetic TextNode

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/MarkdownComposer.tsx

@@ -284,10 +284,10 @@ function ToolbarPlugin({ disabled }: { disabled: boolean }) {
         p.selectEnd();
       } else {
         const code = $createCodeNode();
-        const textNode = $createTextNode(topElement.getTextContent());
-        code.append(textNode);
+        const text = topElement.getTextContent();
+        if (text) code.append($createTextNode(text));
         topElement.replace(code);
-        textNode.select(0, 0);
+        code.select();
       }
     });
   }, [editor]);