feat(ui): a shell a phone can be held in

Plan 016 B2, phase 1. Below 600px the grid drops its sidebar column,
`bottom-nav` becomes the primary navigation, and the shell fits the
viewport instead of scrolling sideways out of it.

600 rather than the sidebar's own 900, because 900 is a laptop and the
answer there is a narrower sidebar, which is still a sidebar. Under 600
there is no room for one at all: 360px of viewport over a 200px nav is
not a layout.

**The tab bar is four destinations and a way to everything else.**
Three to five is where touch targets stop being thumb-sized -- eleven
over 360px is 32px each -- so the four are the ones plan 016's subset
says a phone is for, and "More" opens the *existing* `app-sidebar` in a
drawer rather than listing the destinations a second time. Two lists is
two places to add the next view to.

That reuse has a cost this found the hard way: a shared component
brings its `data-testid`s with it, so rendering the drawer's sidebar
unconditionally put a second `nav-home` (and ten siblings) in the DOM
and **failed 30 existing specs** with "resolved to 2 elements" -- on a
desktop viewport, where this element is `display: none` and the drawer
can never open. It renders only while the drawer is open, and the
component test asserts the absence, because the failure is invisible
from inside the component and lands in files nobody touched.

**What made the shell overflow was minimums, not padding.** Measured at
360px: the body was 652px wide, because a `min-width` in a flex row is
a hard floor and a grid item's implicit minimum is its content. So
`min-width: 0` on the boxes between the viewport and the content, and
each component stands its own non-essential parts down in its *own*
stylesheet -- search-bar's 200px floor, job-indicator's label (the
visible one; the live region that announces it is untouched),
audio-player's seek bar and volume. A media query inside a shadow root
is answered by the viewport, so this is the component saying what it
drops rather than the shell reaching in.

Volume goes because the hardware keys own it on a phone, which is the
same reason mediacontrols' Android handler implements no volume
callback. Seeking goes because 4px is not a thumb target; it belongs to
the full-screen now-playing view, which is the next phase.

An existing spec therefore asserts the opposite of what it did:
layout-overflow's 320px case used to require that the 464px behind
`overflow: hidden` could be *scrolled to*, which was the remedy
available while the shell had one layout. It reflows now -- 320px in a
320px viewport, exactly -- and reflow is what WCAG 1.4.10 asked for.

frontend/index.css

@@ -41,6 +41,90 @@ body {
     overflow-y: hidden;
 }
 
+/* ---------------------------------------------------------------
+   The phone shell (plan 016 B2).
+
+   600px, not the sidebar's 900: 900 is a *laptop* and the response to
+   it is a narrower sidebar, which is still a sidebar.  Below 600 there
+   is no room for one at all -- 360px of viewport over a 200px nav is
+   not a layout -- so the navigation moves to the bottom, where a thumb
+   is, and the eleven-item list moves into `bottom-nav`'s drawer.
+
+   The grid loses its sidebar column rather than hiding the element in
+   place: a named area with nothing in it still reserves its track.
+   --------------------------------------------------------------- */
+@media (max-width: 599px) {
+    body {
+        grid-template:
+            "top-bar" 3.25em
+            "main-panel" 1fr
+            "bottom-bar" auto
+            "bottom-nav" auto
+            / 1fr;
+        /* Nothing may scroll sideways here.  On a desktop the shell is
+           allowed to overflow a zoomed-in window (a11y.21 above); a
+           phone *is* the small viewport, so the shell has to fit it. */
+        overflow-x: hidden;
+    }
+
+    body div.sidebar {
+        display: none;
+    }
+
+    bottom-nav {
+        grid-area: bottom-nav;
+    }
+
+    /* The 2em gutters are half a thumb each at this width, and the
+       subtitle is already gone from 900 down.
+
+       `min-width: 0` is the load-bearing half. A grid item's implicit
+       minimum is `auto` -- its content -- so a header whose children
+       ask for 580px makes the *body* 580px wide inside a 360px
+       viewport, and `overflow-x: hidden` then hides the right-hand
+       third of the app rather than fitting it. Every box between the
+       viewport and the content that must shrink needs this. */
+    .top-bar {
+        padding-left: 0.75em;
+        padding-right: 0.75em;
+        gap: 0.5em;
+        min-width: 0;
+        overflow: hidden;
+    }
+
+    .content-area,
+    .main-panel,
+    .bottom-bar {
+        min-width: 0;
+    }
+
+    .title {
+        font-size: 1.1em;
+    }
+
+    /* The search box is the one header control worth its width; the
+       library filter is a rarely-changed setting and reachable from
+       the drawer's Settings. */
+    .top-bar library-filter {
+        display: none;
+    }
+
+    .top-bar search-bar {
+        flex: 1 1 auto;
+        min-width: 0;
+    }
+}
+
+/* Above the phone breakpoint the tab bar does not exist.  It is in the
+   markup unconditionally and eagerly, for the reason notification-host
+   is: navigation that has to fetch a chunk before it can navigate is
+   not navigation. */
+@media (min-width: 600px) {
+    bottom-nav {
+        display: none;
+    }
+}
+
 p {
     margin: 0;
     /* I want to set paragraph margins myself */
@@ -130,6 +214,21 @@ body div.sidebar {
     contain: layout style paint;
 }
 
+/* The transport at phone width: art, title and the controls, with the
+   seek bar and volume dropped by the components that own them.  The
+   desktop's three fixed columns start with a 320px now-playing, which
+   at 360px of viewport leaves the controls 40px. */
+@media (max-width: 599px) {
+    .bottom-bar {
+        grid-template-columns: minmax(0, 1fr) auto auto;
+        gap: 0.25em;
+    }
+
+    .bottom-bar audio-player {
+        margin: 0.25em;
+    }
+}
+
 .bottom-bar {
     grid-area: bottom-bar;
     padding: 0.25em;