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.html

@@ -81,6 +81,16 @@
             </button>
         </div>
     </footer>
+    <!-- How far through the song we are, on the border between the two
+         bars (#58). The shell's element rather than either bar's:
+         they are separate components stacked in this grid, so a line
+         on the border between them is a row of it, and neither one has
+         to reach into the other's box for two pixels. It renders
+         nothing above 600px and nothing with no track, is `aria-hidden`
+         (Now Playing's seek bar is what announces the position) and
+         takes no pointer events at all -- a thin line that sometimes
+         seeks is worse than one that never does. -->
+    <player-progress-line></player-progress-line>
     <!-- The phone's primary navigation, hidden above 600px by
          index.css.  Eager rather than a chunk, for the reason
          notification-host is: it is the only way to move around the