fix(tui): prevent whitespace-only prompt from being submitted #32631

Summary

The TUI prompt can be submitted when it contains only whitespace (spaces, tabs, newlines), which sends an effectively empty message to the session.

Root Cause

In packages/tui/src/component/prompt/index.tsx, the submit guard checks:

if (!store.prompt.input) return false

This only blocks a completely empty string. A string of whitespace characters (" ", "\t", etc.) is truthy and passes the check, resulting in a blank user message being sent.

Fix

Change the guard to trim whitespace before checking:

if (!store.prompt.input.trim()) return false

The trimmed value is already computed two lines below for command detection, so this is a minimal, safe change.

Steps to Reproduce

1. Open the TUI.
2. Type one or more spaces in the prompt.
3. Press Enter.
4. Observe that a blank user message bubble appears and the session receives an empty prompt.

Expected Behavior

Pressing Enter on a whitespace-only prompt should do nothing (same behavior as an empty prompt).