feat(shell): show background jobs in the phone's layout, not a popover

The header indicator is a disclosure anchored to a bar 3.25em tall on a
screen 439 CSS px tall, and it was reported as unreadable behind other
UI. Background work is the one thing a phone should not make you open
something to see, and #57 deletes the bar it hangs from and is blocked
on it having somewhere else to live. Below 600px the indicator stands
down and <job-band> takes over.

It is the existing job-panel at `kinds="*"`, so pause, cancel, Details
and the log come along, and so does applyJobControl.

**It is in the layout, not over it**, and that was measured rather than
assumed. The first version put the panel in notification-host's fixed
band: it renders correctly, sits on top and stays inside the viewport,
and is unusable -- at 424x439 a compact panel showing two jobs is
~216px of a 439px screen, drawn over the content and swallowing every
tap under it. Four e2e specs caught it, and none of them was about
jobs: two phone-shell journeys and the header's action menu, all
failing on clicks the band was intercepting. As a grid row above the
main panel it pushes instead, which is #24's one sentence deciding a
layout question -- a band that hides the app to say the app is busy has
traded the popover's fault for a worse one.

It renders nothing above 600px, from matchMedia rather than a media
query, because that decides whether the element exists: Settings
already holds four job-panels and a fifth answering for every kind is
bottom-nav's "resolved to 2 elements" trap again. index.css keeps it
display:none off the phone for a second reason -- an in-flow grid child
with no named area is auto-placed into one of the shell's rows, which
is what the skip link is absolutely positioned to avoid.

top-bar-fit's 390px case asserted the indicator was up, so that it
could not pass by measuring the idle case under another name. At phone
width it is now deliberately away, so the assertion takes the other
branch of the same rule -- the indicator is hidden, the band has the
row, and the bar still has nothing hanging out of it -- rather than
the width being quietly dropped from the list.

The report's own symptom is deliberately not asserted anywhere: it did
not reproduce in this tier. Measured at 424x439 the popover was neither
clipped nor covered, so a spec claiming a stacking fix would be
asserting something that was never true here. The spec says so.

Closes #62

frontend/index.html

@@ -30,6 +30,14 @@
         <search-bar></search-bar>
         <job-indicator></job-indicator>
     </header>
+    <!-- The phone's view of background work (#62): below 600px the
+         indicator above stands down and its rows appear here instead,
+         in the layout rather than over it.  `display: none` above that
+         width in index.css, which is also what keeps it out of the
+         desktop grid -- an in-flow child with no named area is
+         auto-placed into one of the shell's rows, which is the trap the
+         skip link is absolutely positioned to avoid. -->
+    <job-band></job-band>
     <div class="sidebar">
         <app-sidebar></app-sidebar>
     </div>