feat(player): draw no volume control where there is no volume

volume-control asks the player whether there is a volume of ours to
control, and renders nothing when there is not.  The decision is in the
control rather than at either mount point because there are two, and
one of them -- the bottom bar's -- lives in index.html, which has no
module scope to make it conditional.

It could not have been a width, and that is the whole design decision.
Every other stand-down rule in this app is keyed on a viewport, because
a width is what a browser can answer and what every tier can test.
This one is a property of the build: keyed on width, an Android tablet
at 600px or more draws the bar's slider over a level the backend has
pinned -- a control that cannot act, on exactly the platform the rule
exists for, which library-status-indicator settled is worse than none.
The same rule is wrong the other way below 600px, where a narrow
desktop window has no hardware keys to fall back on.  index.css keeps
its phone rule, which is now about room and says so.

Rendering nothing and hiding the host are both needed and are separate
assertions: an empty shadow root is what stops a by-role or positional
query finding a button that cannot act, and :host([hidden]) is what
stops the element taking a flex item's worth of the transport.  The
host rule has to be written down, since :host { display: inline-flex }
outranks the UA's [hidden].

Measured at 424x439 by flipping the constant and rebuilding: the album
art goes 39px to 68px and the transport 172px to 143px -- 29px, being
the 21px control plus the 8px gap a hidden box stops drawing.  The
bar's centring is unaffected, since #23's outer columns are the same
min() expression rather than content-sized.

volume-ownership.test.ts is the tier that can exercise the Android
rendering, on an ordinary Linux runner, because the predicate is a
stubbable backend answer.  Both of its tests were confirmed to fail on
the build before this.

Closes #64
Closes #172

frontend/index.css

@@ -522,16 +522,20 @@ body div.sidebar {
     }
 
     /* Volume stands down here whatever the setting says, because this
-       is about room and about the platform rather than about
-       preference: the hardware keys own volume on a phone, which is
-       also why mediacontrols' Android handler implements no volume
-       callback. It moved from `audio-player`'s own media query when
-       #42 moved the control into the bar — same rule, and now stated
-       where the element actually is.
+       is about room: five controls and a slider do not fit a 360px
+       bar, and the full-screen now-playing view is where seeking and
+       volume go on a phone. It moved from `audio-player`'s own media
+       query when #42 moved the control into the bar — same rule, and
+       now stated where the element actually is.
 
-       `.bottom-bar volume-control`, not the one in
-       `now-playing-view`: that view is the phone's transport and is
-       where a slider does belong. */
+       **This rule used to carry the platform argument too, and no
+       longer does** (#64). "The hardware keys own the volume" is not a
+       width: it is false of a narrow desktop window and true of an
+       Android tablet, which this selector gets backwards both ways.
+       The player answers it now — `SystemOwnsVolume` — and
+       `volume-control` renders nothing when it is true, at every
+       width and in both of its mount points. What is left here is the
+       question a stylesheet can actually answer. */
     .bottom-bar volume-control {
         display: none;
     }