Add a config value for the Space hold threshold, wire it into both TUI composer implementations, and document the default behavior.
Also mirror the voice transcription feature key and Space-hold delay in config-schema, and keep placeholder replacement available on Linux for the voice path.
(cherry picked from commit 9fa36434d566153cc69d2fabac095e313d93be51)
(cherry picked from commit 749a0f42619c8cb66514c0bce220b775c10916e3)