feat(ui): the full-screen now playing a phone needs

Plan 016 B2, phase 2. Phase 1 took the seek bar and the volume out of
the phone's bottom bar -- 4px of height is not a thumb target, and a
phone's volume belongs to its hardware keys -- and promised them a
full-screen view. This is it, reached from a button over the mini
player's cover art.

**It composes the transport rather than reimplementing it.** The same
`seek-bar`, `player-controls` and `volume-control` the desktop bar
uses; a phone layout that copies them is a second transport to fix
every bug in, and the seek bar in particular carries interpolation
rules that took a plan of their own to get right. The seek bar
thickens its own track below the breakpoint, in its own stylesheet,
because the track size lives on a wa-slider inside its shadow root
where a custom property from the host cannot reach.

**It is a detail view, not a primary one.** It is somewhere you go and
come back from, so index.ts pushes the current view and Back pops it --
which is also why it is not a fifth tab: a tab you cannot leave by
pressing it again is not a tab.

Two things came from reading a screenshot rather than from a failing
test, and both were invisible to assertions that were individually
correct.

**The mini player was still under the full-screen view**, repeating it
in 4em of an 844px phone. index.css hides the bottom bar while
`#main-content[data-active-view="now-playing"]`, through `:has()`
rather than a class toggled from index.ts, because the active view is
already published as an attribute. That takes the queue button with it,
so the view carries its own.

**And phase 1's shell rules had never applied.** A media query adds no
specificity, and the phone block sat above the plain rules it meant to
override, so at 390px the header kept its 2em gutters (32px), its 16px
gap and its 24px title, and the bottom bar kept a fixed 320px first
column. Nothing failed: the shell fits because of `min-width: 0` and
each component's own media query, which live in their own stylesheets
and have no later rule to lose to -- so what was dead was exactly the
cosmetic half no assertion looks at. The phone rules are one section at
the end of the file now, and it says why it is last. Measured after:
12px, 8px, 17.6px, `154px 187px 33px`.

e2e/specs/phone-shell.spec.ts

@@ -112,6 +112,41 @@ test.describe('the shell on a phone', () => {
     });
   }
 
+  test('opens the full-screen now playing, and comes back', async ({ app }) => {
+    // Something has to be playing for the mini player to be a way in.
+    await app.getByTestId('tab-tracks').click();
+    await expect(app.getByTestId('main-content'))
+      .toHaveAttribute('data-active-view', 'tracks');
+
+    await app.locator('track-list .track-row').first().dblclick();
+    await expect(app.getByTestId('now-playing-title')).not.toBeEmpty();
+
+    await app.getByTestId('open-now-playing').click();
+
+    await expect(app.getByTestId('main-content'))
+      .toHaveAttribute('data-active-view', 'now-playing');
+
+    // The seek bar and volume that phase 1 took out of the bottom bar
+    // are here, and they are the *same* components -- this view
+    // composes the transport rather than reimplementing it.
+    await expect(app.locator('now-playing-view seek-bar')).toBeVisible();
+    await expect(app.locator('now-playing-view volume-control')).toBeVisible();
+
+    // Back goes where the user came from, through the nav stack.
+    await app.getByTestId('npv-back').click();
+    await expect(app.getByTestId('main-content'))
+      .toHaveAttribute('data-active-view', 'tracks');
+  });
+
+  test('offers no way in on a desktop, where the bar is whole', async ({ app }) => {
+    await app.setViewportSize({ width: 1440, height: 900 });
+
+    // The button exists in the markup at every size; CSS decides. If
+    // this becomes visible on a desktop it is a 48px hit target over
+    // the cover art, swallowing the clicks that open the preview.
+    await expect(app.getByTestId('open-now-playing')).toBeHidden();
+  });
+
   test('keeps the transport, minus what a thumb cannot use', async ({ app }) => {
     // The player bar stays: this is a music player, and what is playing
     // has to be visible and pausable from every view.