Add monospace fallback to JetBrains Mono fonts

When users block web fonts, code was rendered in the browser's default
sans-serif font because the JetBrains Mono code font tokens had no
generic fallback. Add `monospace` as a fallback so code stays
monospaced.

Fixes: 8942cea33a

src/lib/appearance.ts

@@ -40,7 +40,7 @@ export const codeFonts: {
 }[] = [
   {
     storedName: "jetbrains",
-    fontFamily: "JetBrains Mono",
+    fontFamily: "JetBrains Mono, monospace",
     displayName: "JetBrains Mono",
   },
   { storedName: "system", fontFamily: "monospace", displayName: "System" },