Screen: switch off by itself when nothing is going on

With `general.screen.off_command` set, the monitor switches off once nothing has
played and nobody has done anything for `auto_off_minutes` (15 by default, 0 for
never, editable in parent mode). "Doing anything" is any intent a person could
have caused: a remote key, a web command, or a key press or tap in the kiosk,
which the page reports at most every 30s via POST /api/screen/activity. The first
such intent after an auto-off runs `on_command`, so the remote wakes the screen
too. The auto-off itself is silent; switches a person asks for still blubb.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/api/client.ts

@@ -98,6 +98,8 @@ export const api = {
   setVolume: (percent: number) => post("/volume", { percent }),
   nudgeVolume: (deltaPercent: number) => post("/volume", { delta_percent: deltaPercent }),
   toggleScreen: () => post("/screen/toggle"),
+  /** Someone is at the kiosk: holds off the screen's idle auto-off. */
+  screenActivity: () => post("/screen/activity"),
 
   settings: () => request<Settings>("/settings"),
   saveSettings: (settings: Settings) =>