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>

python-backend/README.md

@@ -216,7 +216,7 @@ copied to the device.
 | `WS /api/ws` | Push only. A snapshot on connect, then a frame per change, plus the position at 2 Hz while playing. |
 | `POST /api/play` | `{album_id, track_index?}` |
 | `POST /api/resume` `/pause` `/next` `/previous` `/seek` | transport |
-| `POST /api/volume` | `{percent}` or `{delta_percent}` |
+| `POST /api/volume` | `{percent}`, `{delta_percent}`, or `{steps}` - the IR remote's step (`volume_increment` device units) per step |
 | `POST /api/remote/press/{digit}` | what the IR remote's number key does - the keyboard's digits |
 | `GET` `PUT /api/remote/mapping` | number key -> album (optionally from one `track_index`) or podcast show |
 | `POST /api/screen/toggle` `/screen/activity` | the kiosk's Del key; its key presses and taps, holding off the idle screen-off |