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.css

@@ -426,6 +426,7 @@ body div.sidebar {
             "jobs-band" auto
             "main-panel" 1fr
             "bottom-bar" auto
+            "progress-line" auto
             "bottom-nav" auto
             / 1fr;
         /* Nothing may scroll sideways here.  On a desktop the shell is
@@ -501,7 +502,8 @@ body div.sidebar {
        expression of the same fact is a second thing to keep in step.
        The view carries its own queue button, because this is where
        that one lived. */
-    body:has(#main-content[data-active-view="now-playing"]) .bottom-bar {
+    body:has(#main-content[data-active-view="now-playing"]) .bottom-bar,
+    body:has(#main-content[data-active-view="now-playing"]) player-progress-line {
         display: none;
     }
 }
@@ -568,8 +570,12 @@ body div.sidebar {
 /* Out of the desktop grid entirely.  `job-band` renders nothing above
    600px anyway, but an in-flow grid child with no named area is
    auto-placed into a row of the shell -- the same trap the skip link is
-   absolutely positioned to avoid. */
-body job-band {
+   absolutely positioned to avoid.  `player-progress-line` (#58) is the
+   same element in the same position for the same reason: below 600px it
+   has a named row, and above it there is no border for it to sit on --
+   the desktop bar carries a real, interactive seek bar. */
+body job-band,
+body player-progress-line {
     display: none;
 }
 
@@ -602,3 +608,21 @@ body job-band {
         background-color: var(--yj-bg-elevated, #343a40);
     }
 }
+
+/* #58.  How far through the song we are, in its own grid row between
+   the two bars -- so the line is *on* the border rather than inside
+   either of them, and in flow rather than over it.  The row is `auto`
+   and the element renders nothing while no track is loaded, so it costs
+   no height at all until there is something to say.
+
+   **This block is below the `display: none` above and has to be**, for
+   the reason the band's rule is: a media query adds no specificity, so
+   `body player-progress-line { display: block }` written before that
+   rule loses to it at equal specificity and the line never appears at
+   any width.  Nothing fails; it is simply not there. */
+@media (max-width: 599px) {
+    body player-progress-line {
+        display: block;
+        grid-area: progress-line;
+    }
+}