fix(ui): make goal description area scrollable in create dialog

Long goal descriptions pushed the Create button below the viewport
with no way to scroll, making it impossible to submit the form. Add
overflow-y-auto and max-h-[50vh] to the description container so it
scrolls within the dialog while keeping the footer visible.

Closes #2631

ui/src/components/NewGoalDialog.tsx

@@ -168,7 +168,7 @@ export function NewGoalDialog() {
         </div>
 
         {/* Description */}
-        <div className="px-4 pb-2">
+        <div className="px-4 pb-2 overflow-y-auto max-h-[50vh]">
           <MarkdownEditor
             ref={descriptionEditorRef}
             value={description}