feat(design): bundle JetBrains Mono for data cells

frontend/src/styles/fonts.css

@@ -1,6 +1,7 @@
 /*
  * Inter Variable — bundled locally via @fontsource-variable/inter.
+ * JetBrains Mono Variable — bundled locally via @fontsource-variable/jetbrains-mono.
  * NO remote CDN / Google Fonts loading at runtime.
- * Forma DJR Micro substitute per DESIGN.md §Note on Font Substitutes.
  */
 @import '@fontsource-variable/inter/index.css';
+@import '@fontsource-variable/jetbrains-mono/index.css';