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/tests/test_web.py

@@ -292,13 +292,29 @@ async def test_volume_steps_clamp_at_the_ends(client: Client, sim: Simulation) -
     assert (await client.get("/api/state")).json()["volume"] == 100
 
 
+async def test_remote_steps_move_by_the_volume_increment(client: Client, sim: Simulation) -> None:
+    """The +/- buttons take the IR remote's step, in device units, not a percentage."""
+    await client.post("/api/volume", json={"percent": 50})
+    await sim.bus.drain()
+    before = sim.player.volume
+    step = sim.app.config.general.volume_increment
+
+    assert (await client.post("/api/volume", json={"steps": 1})).status_code == 204
+    await sim.bus.drain()
+    assert sim.player.volume == before + step
+
+    await client.post("/api/volume", json={"steps": -2})
+    await sim.bus.drain()
+    assert sim.player.volume == before - step
+
+
 async def test_the_device_volume_range_is_never_exposed(client: Client) -> None:
     state = (await client.get("/api/state")).json()
     assert "volume_min" not in state
     assert "volume_max" not in state
 
 
-async def test_volume_needs_one_of_the_two_fields(client: Client) -> None:
+async def test_volume_needs_one_of_the_fields(client: Client) -> None:
     assert (await client.post("/api/volume", json={})).status_code == 422