fix: prevent attachment button click from triggering form submit

Clicking the attachment button in the web UI inadvertently
triggers a form submit, sending the current input text before
the file dialog opens.

Add preventDefault() and stopPropagation() to the click handler
to ensure only the file dialog is opened without side effects.

Fixes #1428

Author: Br1an67

web/src/features/chat/components/attachment-button.tsx

@@ -5,7 +5,13 @@ export function AttachmentButton() {
   const attachments = usePromptInputAttachments();
 
   return (
-    <PromptInputButton onClick={() => attachments.openFileDialog()}>
+    <PromptInputButton
+      onClick={(e: React.MouseEvent) => {
+        e.preventDefault();
+        e.stopPropagation();
+        attachments.openFileDialog();
+      }}
+    >
       <PaperclipIcon className="size-4" />
     </PromptInputButton>
   );