Hide voice mode placeholder while the user is typing

[meganrogge]

From Karthik Nadig (@karthiknadig)

When voice mode is active, the chat input shows a listening or barge-in placeholder. If the user starts typing, that placeholder should be hidden so it does not compete with the typed input.

The placeholder should become visible again after the input is cleared, as long as the voice state still calls for it.

Acceptance criteria:

• Hide the listening placeholder while the chat input contains typed text.
• Hide the barge-in placeholder while the chat input contains typed text.
• Restore the applicable placeholder when the input becomes empty again.