feat(web): opt-in setting to start new threads in background#23915

docs: spell out the multiline case for starting new threads in background

fix(web): label Option on macOS in the start-in-background hint

fix(web): keep the send chord starting in background with a modifier send shortcut

chore(web): trim comments on the start-in-background setting

feat(web): start new threads in background from settings

feat(web): add start-new-threads-in-background send rule