fix: move file input inside dialog to prevent backdrop close killing it

The hidden file input was outside the inner dialog div (which has
stopPropagation) but inside the backdrop div (which calls onClose).
When the OS file picker closed, the backdrop click handler fired,
unmounting the modal before FileReader could complete.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/quickstart/WelcomeModal.tsx

@@ -107,8 +107,6 @@ export function WelcomeModal({
             onWorkflowGenerated={handleWorkflowSelected}
           />
         )}
-      </div>
-
         {/* Hidden file input for loading workflows */}
         <input
           type="file"
@@ -118,5 +116,6 @@ export function WelcomeModal({
           className="hidden"
         />
       </div>
+    </div>
   );
 }