feat(player): slim the phone's mini player to three controls

Shuffle, repeat and the queue button leave the phone's bottom bar.
They are not gone: all three are on the full-screen Now Playing view,
one tap away through the mini player's art, which is the "reachable
only from Now Playing" this issue asks for. #55 is what makes the queue
half safe -- it is a screen with an entry in the back stack now, rather
than a panel with no way out but the button being removed here.

Removing a control is only allowed because it is still reachable, which
is plan 018's matrix promise, so that is what the spec walks rather
than counting buttons. It found that the route did not exist in the
state that matters: `now-playing` renders two branches and the no-track
one had no `.expand` button on its placeholder, so with nothing loaded
there was no way to the full-screen view at all -- and once the queue
button left the bar, no way to the queue. The queue is persisted across
restarts, so "tracks queued, nothing playing" is a state the app
launches into, not a corner.

The favourite stays on the bar and was 18x14px, the smallest control in
the app, against the 48x48 art beside it.

One CSS trap, because it failed silently. The phone block is last in
index.css on purpose -- a media query adds no specificity -- but the
rule it overrides here is written *nested* inside `.bottom-bar`, so it
builds to a descendant selector one class more specific and a bare
`#queue-button` lost to it. Being last is not enough when the thing
above is more specific.

Closes #59

frontend/index.css

@@ -535,6 +535,30 @@ body div.sidebar {
     .bottom-bar volume-control {
         display: none;
     }
+
+    /* The queue leaves the phone's bar (#59), because #55 made it a
+       screen with an entry in the back stack and Now Playing already
+       carries its own button for it. The route is the mini player's
+       art -> Now Playing -> the queue, which is the "reachable only
+       from Now Playing" this issue asks for.
+
+       This is allowed to remove a control only because the control is
+       still reachable: plan 018's matrix promises that no action is
+       ever unreachable at any supported size, and that promise is what
+       `phone-transport.spec.ts` asserts rather than the button count.
+
+       **`.bottom-bar #queue-button`, not `#queue-button`**, and that is
+       not decoration. The rule this overrides is written *nested*
+       inside `.bottom-bar`, so it builds to a descendant selector one
+       class more specific than it looks in the source -- and a bare
+       `#queue-button` here loses to it, media query or not. Being last
+       in the file is not enough when the thing above is more specific,
+       which is the same lesson as this section's own header one level
+       down: nesting adds specificity the source does not show, and the
+       failure is silent (the button simply stayed). */
+    .bottom-bar #queue-button {
+        display: none;
+    }
 }
 
 /* Out of the desktop grid entirely.  `job-band` renders nothing above