frontend: fix markdown editor overflowing its container (#2219)

Long text in the Milkdown editor would overflow the card boundary and
break the page layout on both agent create and edit pages.

Three CSS properties fix this:
- min-width: 0 on the wrapper lets it shrink in flex/grid contexts
- overflow-wrap: break-word breaks long words at the container edge
- overflow: hidden as a hard containment boundary

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: birdayz

frontend/src/components/ui/markdown-editor/markdown-editor.css

@@ -1,8 +1,15 @@
 /* Milkdown Editor Styles */
 
+.markdown-editor-wrapper {
+  min-width: 0;
+}
+
 .markdown-editor-wrapper .milkdown {
   min-height: 80px;
   padding: 0.5rem 0.75rem;
+  overflow-wrap: break-word;
+  word-break: break-word;
+  overflow: hidden;
 }
 
 .markdown-editor-wrapper .milkdown .editor {