Add volume management to AVR

README.md

@@ -6,7 +6,7 @@ A phone-sized web remote for a multi-room HEOS system, meant to be added to the
 
 Everything fits on one screen:
 
-- **Volume** up/down for each room. A tap lands on the next multiple of `VOLUME_STEP` — from 23 it goes to 25, not 28 — so the levels stay round. Hold to keep moving, or drag the level along its bar to set it outright.
+- **Volume** up/down for each room and for the AVR. A tap lands on the next multiple of `VOLUME_STEP` — from 23 it goes to 25, not 28 — so the levels stay round. Hold to keep moving, or drag the level along its bar to set it outright.
 - **Navigate** either room (play / pause / prev / next), while it is playing Spotify
 - **Group** either room with the AVR. the AVR is always the host, so its sound takes over whatever joins it. A room that joins moves _into_ the Home Cinema card, so one glance says what is playing together
 - **Ungroup** either room again, or all of them at once
@@ -138,7 +138,7 @@ Used by the interface:
 | --- | --- |
 | `GET /api/state` | everything the UI draws, in one call |
 | `GET /api/targets` | every player and group HEOS can see |
-| `POST /api/volume` | `{"target": "lego_room", "steps": 1}` — taps, snapped to `VOLUME_STEP`. Also takes `delta` (raw points) or `level` (absolute) |
+| `POST /api/volume` | `{"target": "lego_room", "steps": 1}` — taps, snapped to `VOLUME_STEP`. Also takes `delta` (raw points) or `level` (absolute). `avr` is a target like any room |
 | `POST /api/mute` | `{"target": "lego_room"}` |
 | `POST /api/playback` | `{"target": "lego_room", "state": "pause"}`, or no `state` to toggle — the Zidoo's film for the AVR (`play` and `pause` only), a room's own stream over HEOS otherwise |
 | `POST /api/skip` | `{"target": "lego_room", "direction": "next"}` — `previous` too |