Snap volume to whole steps, and hide deleted AVR sources

A tap now moves to the next multiple of VOLUME_STEP rather than adding
it, so 23 goes to 25 and 25 goes to 30 and the levels stay round. The
panel counts taps and lets the speakers do the rounding from whatever
level they are actually at, since the phone's copy can be seconds old;
the same rule is mirrored in JS so the optimistic number never has to
correct itself when the reply lands.

The input picker also asks the AVR which sources are still switched on
(SSSOD ?) and leaves out the ones deleted in its setup menu. Sources it
does not mention are kept, so a model that ignores the command shows its
whole list rather than nothing; deleted sources also keep their names, in
case the AVR is sitting on one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

config.py

@@ -55,11 +55,14 @@ HOST_KEY = "avr"
 ROOM_KEYS = ["home400", "living_room_group"]
 
 # --- Behaviour --------------------------------------------------------
-# How many HEOS volume points one tap of +/- moves.
+# The grid the volume buttons snap to. A tap moves to the next multiple of
+# this rather than adding it, so at 5 a level of 23 goes to 25, not 28.
 VOLUME_STEP = 5
 
-# Limit the input picker to the sources you actually use, by their SI
-# code (see GET /api/avr/inputs for the full list). Empty = show all.
+# Sources you deleted in the AVR's setup menu are hidden from the picker
+# automatically. This narrows it further to the ones you actually use, by
+# their SI code (see GET /api/avr/inputs), and sets their order in the
+# list. Empty = every source the AVR still has switched on.
 AVR_INPUT_CODES = []
 
 # Shown as the app's name on the iOS home screen.