Voice mode: add toggle option (tap-to-start/tap-to-stop)

Feature request

Currently /voice mode requires holding Space for the entire duration of recording. This defeats the hands-free benefit since you're stuck at the keyboard anyway.

Requested: Add a toggle mode — tap Space to start recording, tap again to stop. The current hold-to-record can remain as the default, but a toggle option would make voice mode actually useful for hands-free use (e.g., describing a problem while looking at hardware, whiteboarding, etc.).

Could be a setting like voiceMode: "hold" | "toggle" or a keybinding config.