fix(ui): restore font-family inherit in base layer for non-monospace editors

Move font-family: inherit into @layer base so it still applies to
non-monospace MDXEditor instances (comments, dialogs) but yields to
Tailwind's font-mono utility for agent instruction editors.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/src/index.css

@@ -319,6 +319,13 @@
   color: inherit;
 }
 
+@layer base {
+  .paperclip-mdxeditor [class*="_placeholder_"],
+  .paperclip-mdxeditor-content {
+    font-family: inherit;
+  }
+}
+
 .paperclip-edit-in-place-content {
   font-size: 0.9375rem;
   line-height: 1.75rem;