Volume: quieter/louder buttons that take the remote's own step

Next to the five volume bars, a − and a + button move the volume by
`volume_increment` device units, the same step as the IR remote's volume keys and
the rotary encoder, via a new `{steps}` form of POST /api/volume. The bars still
jump to a percentage.

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

web/src/api/client.ts

@@ -97,6 +97,8 @@ export const api = {
   seek: (position: number) => post("/seek", { position }),
   setVolume: (percent: number) => post("/volume", { percent }),
   nudgeVolume: (deltaPercent: number) => post("/volume", { delta_percent: deltaPercent }),
+  /** One step of the IR remote's volume keys per unit, in the device's own increment. */
+  stepVolume: (steps: number) => post("/volume", { steps }),
   toggleScreen: () => post("/screen/toggle"),
   /** Someone is at the kiosk: holds off the screen's idle auto-off. */
   screenActivity: () => post("/screen/activity"),