feat(player): show progress on the phone's bar border

#59 took the seek bar off the phone's transport, so the one thing a
mini player is expected to say without being opened -- how far through
the song it is -- had nowhere left to be said.

It is the shell's element and its own 2px grid row between `bottom-bar`
and `bottom-nav`, because those two are separate components and either
one drawing the line means reaching into the other's box. The fill is
`scaleX()` off the same `PlaybackPositionChanged` the seek bar renders,
with the same `trackChangeId`/`seq` guards and an interval that only
interpolates *between* reports -- never its own clock, which is the
rule that exists because a local counter drifted 30 s away from the
backend across four keyboard seeks.

It is `aria-hidden` and takes no pointer events at any depth: Now
Playing's seek bar is what announces the position, and a 2px strip on
the top edge of the tab bar is exactly where a thumb aiming at a tab
lands. It renders nothing above 600px, from `matchMedia` rather than a
media query, because a stylesheet cannot stop a 1 Hz interval running
for the life of every desktop session about a line nobody can see.

Its phone rule is at the foot of index.css beside `job-band`'s, not in
the phone block above: a media query adds no specificity, so a
`display: block` written before the `display: none` that takes it out
of the desktop grid loses to it and the line never appears at all.

Closes #58

frontend/index.ts

@@ -21,6 +21,10 @@ import '@components/audio-player/audio-player.ts';
 // In the bar rather than inside `audio-player` since #42, so the shell
 // is what has to register it.
 import '@components/audio-player/volume-control/volume-control.ts';
+// The phone's progress line (#58), on the border between the mini
+// player and the tab bar. In the shell for the same reason the volume
+// is, and eager because it is part of the bottom bar's first paint.
+import '@components/audio-player/progress-line/progress-line.ts';
 import '@components/track-list/track-list.ts';
 import '@components/now-playing/now-playing.ts';
 import '@components/sidebar/app-sidebar.ts';