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.

.planning/plans/pending/016-android-feature-parity.md

@@ -246,6 +246,13 @@ view's template is two templates to fix every bug in. Where a view
 cannot serve both, the split belongs at the chunk boundary that already
 exists.
 
+Phase 1 followed that rule and found its cost: reusing `<app-sidebar>`
+inside the drawer means reusing its `data-testid`s too, and a second
+copy standing by in the DOM broke 30 specs that had nothing to do with
+the phone. The rule holds — a second list of destinations would be
+worse — but a shared component must be rendered only when it is wanted,
+and the guard belongs in a test that names the reason.
+
 ## What is worth doing regardless of that decision
 
 Cheap, independently useful, and each unblocks measurement:
@@ -308,8 +315,14 @@ places had to agree — `abiFilters`, the Makefile's `android:package`
 anchor is what stops it also matching the fat APK's line. Adding the
 ABI back, if modernc ever fixes `Xlstat64`, is those same three edits.
 
-**B2, the desktop shell.** The largest remaining piece, and the scope
-is now decided — see "The phone gets a subset" below.
+**B2, the desktop shell.** Scope decided (below) and **phase 1 is
+done**: the shell itself. Below 600px the sidebar column is gone,
+`<bottom-nav>` is the primary navigation, and the shell fits 320px
+exactly — measured, from 652px in a 360px viewport before. What is left
+is the *views*: a full-screen now-playing (which is where seeking and
+volume went), long-press for the context menus that are right-click
+today, and the track list's resizable columns, which are a pointer
+feature with no touch equivalent.
 
 **B3/B4** are unchanged, and B3 is now *possible* where it was not:
 with all-files access, `tagwriter` can write in place.