style: remove default border and spacing from pre elements

- Remove default border, padding, and margin from pre tags
- Apply background, border, and padding only to highlighted code blocks (pre:has(code.hljs))
- Keeps overflow-x: auto for all pre elements
- Aligns with previous behavior before syntax highlighting was added

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ericyangpan

src/app/[locale]/globals.css

@@ -111,14 +111,17 @@ strong {
 
 /* Code block styling */
 pre {
+  overflow-x: auto;
+}
+
+/* Only apply background and border for highlighted code blocks */
+pre:has(code.hljs) {
   background-color: #f6f8fa;
   border: 1px solid var(--color-border);
   padding: 1.25rem;
-  margin: 1.5rem 0;
-  overflow-x: auto;
 }
 
-[data-theme="dark"] pre {
+[data-theme="dark"] pre:has(code.hljs) {
   background-color: #161b22;
   border-color: var(--color-border);
 }