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`.

frontend/src/components/now-playing/now-playing.ts

@@ -146,6 +146,41 @@ export class NowPlaying extends LitElement {
       position: relative;
     }
 
+    /* The phone's way into the full-screen now-playing view (016 B2
+       phase 2). It sits over the cover art rather than being a
+       thirteenth control in a 360px bar, and it is a *button* rather
+       than a click handler on the art because it is an action with a
+       name -- the art itself is decorative and the title beside it
+       already navigates somewhere else (the catalog page).
+
+       CSS owns whether it exists, the same way it does for bottom-nav:
+       there is no viewport check in the component. */
+    .expand {
+      display: none;
+    }
+
+    @media (max-width: 599px) {
+      .expand {
+        position: absolute;
+        inset: 0;
+        display: block;
+        width: 100%;
+        height: 100%;
+        padding: 0;
+        background: none;
+        border: none;
+        border-radius: 4px;
+        cursor: pointer;
+        /* The art shows through; this is a target, not a picture. */
+        color: transparent;
+      }
+
+      .expand:focus-visible {
+        outline: 2px solid var(--yj-accent, #ffd43b);
+        outline-offset: 2px;
+      }
+    }
+
     .cover-preview-panel {
       width: 500px;
       height: 500px;
@@ -377,6 +412,13 @@ export class NowPlaying extends LitElement {
       <div class="sr-only" role="status" aria-live="polite">${announcement}</div>
       <div class="now-playing">
         <div class="cover-art-wrapper">
+          <button
+            type="button"
+            class="expand"
+            data-testid="open-now-playing"
+            aria-label="Open now playing"
+            @click=${this.openNowPlaying}
+          ></button>
           <div
             class="cover-art"
             @mouseenter=${this.handleCoverMouseEnter}
@@ -485,6 +527,15 @@ export class NowPlaying extends LitElement {
     `;
     }
 
+    /** Open the full-screen view. Phone only; see `.expand`. */
+    private openNowPlaying = () => {
+        this.dispatchEvent(new CustomEvent('navigate', {
+            detail: { view: 'now-playing' },
+            bubbles: true,
+            composed: true,
+        }));
+    };
+
     // ===================================================================
     // SCROLL LOGIC
     // ===================================================================