Fix workflow Preview run blockers and multi-artifact chips

Make the doc-driven Preview flow usable and surface every produced artifact:
- A Preview submit with attachments but no text now runs the workflow (opt-in on
  MessageInput; no filler text is injected), so a doc-driven workflow can be run
  by attaching its documents.
- The builder's global Escape no longer tears down the whole Preview/drawer, and
  no longer fires from inside text fields.
- The Preview drawer has a visible close button, and the upload toast moves off
  the attach/send controls (to the left) while the Preview is open.
- A turn that produces multiple artifacts now renders one chip per artifact
  (labelled by tool, indexed when several), instead of exposing only the last.

frontend/src/components/MessageInput.tsx

@@ -293,6 +293,10 @@ type MessageInputProps = {
   showSourceButton?: boolean;
   showToolButton?: boolean;
   autoFocus?: boolean;
+  // Opt-in: enable send with empty text when there are completed
+  // attachments (used by doc-driven workflow runs). Normal chat leaves this
+  // unset, preserving the text-required behavior.
+  allowSendWithoutText?: boolean;
 };
 
 export default function MessageInput({
@@ -301,6 +305,7 @@ export default function MessageInput({
   showSourceButton = true,
   showToolButton = true,
   autoFocus = true,
+  allowSendWithoutText = false,
 }: MessageInputProps) {
   const { t } = useTranslation();
   const [value, setValue] = useState('');
@@ -1467,13 +1472,21 @@ export default function MessageInput({
     setIsSourcesPopupOpen(false);
   };
 
+  // When ``allowSendWithoutText`` is set, an attachment-only submit is
+  // permitted as long as at least one attachment has finished processing.
+  const hasCompletedAttachment = attachments.some(
+    (attachment) => attachment.status === 'completed',
+  );
+  const hasSubmittableContent =
+    Boolean(value.trim()) || (allowSendWithoutText && hasCompletedAttachment);
+  const canSubmit =
+    hasSubmittableContent &&
+    !loading &&
+    recordingState !== 'recording' &&
+    recordingState !== 'transcribing';
+
   const handleSubmit = () => {
-    if (
-      value.trim() &&
-      !loading &&
-      recordingState !== 'recording' &&
-      recordingState !== 'transcribing'
-    ) {
+    if (canSubmit) {
       onSubmit(value);
       setValue('');
       if (isTouch) {
@@ -1635,19 +1648,11 @@ export default function MessageInput({
               onClick={handleSubmit}
               aria-label={t('send')}
               className={`ml-auto h-7 w-7 shrink-0 rounded-full transition-colors duration-300 ease-in-out sm:h-9 sm:w-9 ${
-                value.trim() &&
-                !loading &&
-                recordingState !== 'recording' &&
-                recordingState !== 'transcribing'
+                canSubmit
                   ? 'bg-primary text-white'
                   : 'bg-muted text-muted-foreground dark:bg-accent dark:text-muted-foreground'
               }`}
-              disabled={
-                !value.trim() ||
-                loading ||
-                recordingState === 'recording' ||
-                recordingState === 'transcribing'
-              }
+              disabled={!canSubmit}
             >
               <SendArrow
                 className="mx-auto my-auto block h-3.5 w-3.5 sm:h-4 sm:w-4"