fix send button disabled when sharing post via DM without text

The submitDisabled check in MessageInput (native) didn't account for
hasEmbed, so the send button stayed disabled when sharing a post
without entering text. The onSubmit handler and MessageComposer
already handled this correctly.

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

src/screens/Messages/components/MessageInput.tsx

@@ -136,7 +136,8 @@ export function MessageInput({
     scrollEnabled: isInputScrollable.get(),
   }))
 
-  const submitDisabled = needsEmailVerification || message.trim().length === 0
+  const submitDisabled =
+    needsEmailVerification || (!hasEmbed && message.trim().length === 0)
 
   const blur = useCallback(() => {
     inputRef.current?.blur()