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]);

src/renderer/styles.css

@@ -147,10 +147,10 @@ textarea {
 }
 
 .markdown-composer-editable {
-  min-height: 40px;
+  min-height: 52px;
   max-height: 200px;
   overflow-y: auto;
-  padding: 8px 48px 8px 16px;
+  padding: 10px 48px 10px 16px;
   font-size: 14px;
   line-height: 1.6;
   color: #f4f4f5;
@@ -165,7 +165,7 @@ textarea {
 
 .markdown-composer-placeholder {
   position: absolute;
-  top: 8px;
+  top: 10px;
   left: 16px;
   font-size: 14px;
   line-height: 1.6;
@@ -227,10 +227,11 @@ textarea {
 
 /* Code block */
 .mc-code-block {
+  display: block;
   background: rgba(24, 24, 27, 0.8);
   border: 1px solid #27272a;
   border-radius: 6px;
-  padding: 8px 12px;
+  padding: 6px 10px;
   font-family: ui-monospace, SFMono-Regular, "SF Mono", Menlo, monospace;
   font-size: 0.9em;
   white-space: pre-wrap;