style(markdown): improve code font-family formatting and add spacing for headings

- Reformatted the font-family declaration for better readability.
- Added margin-top and margin-bottom spacing for h1, h2, and h3 headings.

Author: Leaf-0w0

src/styles/markdown.css

@@ -39,7 +39,19 @@
   code {
     @apply bg-(--inline-code-bg) text-(--inline-code-color) px-1 py-0.5 rounded-md overflow-hidden;
 
-    font-family: var(--ec-codeFontFml, "JetBrains Mono Variable", "JetBrains Mono", ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace);
+    font-family: var(
+      --ec-codeFontFml,
+      "JetBrains Mono Variable",
+      "JetBrains Mono",
+      ui-monospace,
+      SFMono-Regular,
+      Menlo,
+      Monaco,
+      Consolas,
+      "Liberation Mono",
+      "Courier New",
+      monospace
+    );
 
     &:before,
     &:after {
@@ -61,6 +73,11 @@
     }
   }
 
+  /* --- 标题间距 --- */
+  :is(h1, h2, h3) {
+    @apply mt-4 mb-2;
+  }
+
   /* --- 列表 --- */
   ul,
   ol {