feat(player): show progress on the phone's bar border

#59 took the seek bar off the phone's transport, so the one thing a
mini player is expected to say without being opened -- how far through
the song it is -- had nowhere left to be said.

It is the shell's element and its own 2px grid row between `bottom-bar`
and `bottom-nav`, because those two are separate components and either
one drawing the line means reaching into the other's box. The fill is
`scaleX()` off the same `PlaybackPositionChanged` the seek bar renders,
with the same `trackChangeId`/`seq` guards and an interval that only
interpolates *between* reports -- never its own clock, which is the
rule that exists because a local counter drifted 30 s away from the
backend across four keyboard seeks.

It is `aria-hidden` and takes no pointer events at any depth: Now
Playing's seek bar is what announces the position, and a 2px strip on
the top edge of the tab bar is exactly where a thumb aiming at a tab
lands. It renders nothing above 600px, from `matchMedia` rather than a
media query, because a stylesheet cannot stop a 1 Hz interval running
for the life of every desktop session about a line nobody can see.

Its phone rule is at the foot of index.css beside `job-band`'s, not in
the phone block above: a media query adds no specificity, so a
`display: block` written before the `display: none` that takes it out
of the desktop grid loses to it and the line never appears at all.

Closes #58

e2e/specs/phone-progress-line.spec.ts

@@ -0,0 +1,122 @@
+import {
+  test,
+  expect,
+  callBinding,
+  resetEvents,
+  waitForEvent,
+  LONG_TRACK,
+  NO_QUEUE_SOURCE,
+} from '../support/fixtures.js';
+import type { Page } from '@playwright/test';
+
+/**
+ * The phone's progress line (#58).
+ *
+ * The component tier already pins what the line *says* — that it
+ * renders the backend's reported position and never a count of its own.
+ * What only a real shell can answer is **where it is**: the issue asks
+ * for a line on the border between the mini player and the tab bar, and
+ * "on the border" is two adjacencies in a grid that no component-level
+ * render has around it.
+ *
+ * It also asserts the line is not there on a desktop, which is the
+ * other half of the same fact: above 600px there is no tab bar for it
+ * to sit on the border of, and the bar carries a real seek bar.
+ */
+type Rect = { x: number; y: number; width: number; height: number };
+
+/** The reference device's real viewport. */
+const DEVICE = { width: 424, height: 439 };
+const DESKTOP = { width: 1280, height: 800 };
+
+async function rectOf(app: Page, selector: string): Promise<Rect | null> {
+  return app.evaluate((sel) => {
+    const el = document.querySelector(sel);
+
+    if (!el) return null;
+
+    const r = el.getBoundingClientRect();
+
+    return { x: r.x, y: r.y, width: r.width, height: r.height };
+  }, selector);
+}
+
+/**
+ * Put the 90-second fixture on and wait for the first position report.
+ *
+ * The long track rather than any track: every other fixture is 2-6
+ * seconds, which is shorter than the time this spec takes to measure
+ * three rectangles.
+ */
+async function play(app: Page): Promise<void> {
+  const tracks = await callBinding<{ FilePath: string; TrackName: string }[]>(
+    app,
+    'library.Library.GetTracks',
+    [0],
+  );
+
+  // `TrackName`, not `Title`: that is what the library model calls it.
+  const long = tracks.find((t) => t.TrackName === LONG_TRACK);
+
+  expect(long, `no fixture track named ${LONG_TRACK}`).toBeTruthy();
+
+  await callBinding(app, 'queue.Queue.Clear');
+  await resetEvents(app);
+  await callBinding(app, 'queue.Queue.SetQueue', [
+    [long!.FilePath],
+    0,
+    false,
+    NO_QUEUE_SOURCE,
+  ]);
+  await waitForEvent(app, 'QueueChanged');
+  await callBinding(app, 'queue.Queue.Play');
+  await waitForEvent(app, 'PlaybackPositionChanged', { timeoutMs: 15_000 });
+}
+
+test.describe('the progress line sits on the border between the bars', () => {
+  test.beforeEach(async ({ app }) => {
+    await app.setViewportSize(DEVICE);
+    await play(app);
+  });
+
+  test('spans the width, between the mini player and the tab bar', async ({
+    app,
+  }) => {
+    const line = await rectOf(app, 'player-progress-line');
+    const bar = await rectOf(app, '.bottom-bar');
+    const nav = await rectOf(app, 'bottom-nav');
+
+    expect(line, 'no progress line on the phone').not.toBeNull();
+    expect(bar).not.toBeNull();
+    expect(nav).not.toBeNull();
+
+    // A border, not a band: 2px, the full width, and touching both.
+    expect(line!.height).toBeCloseTo(2, 0);
+    expect(line!.width).toBeCloseTo(bar!.width, 0);
+    expect(line!.y).toBeCloseTo(bar!.y + bar!.height, 0);
+    expect(nav!.y).toBeCloseTo(line!.y + line!.height, 0);
+  });
+
+  /**
+   * It is 2px on the top edge of the tab bar, which is exactly where a
+   * thumb aiming at a tab lands. A line that sometimes seeks is worse
+   * than one that never does, so it must take no part in hit testing
+   * at all.
+   */
+  test('takes no taps', async ({ app }) => {
+    const line = await rectOf(app, 'player-progress-line');
+
+    const hit = await app.evaluate(
+      ({ x, y }) => document.elementFromPoint(x, y)?.tagName ?? '',
+      { x: line!.x + line!.width / 2, y: line!.y + 1 },
+    );
+
+    expect(hit).not.toBe('PLAYER-PROGRESS-LINE');
+  });
+
+  test('is not there on a desktop', async ({ app }) => {
+    await app.setViewportSize(DESKTOP);
+
+    await expect(app.locator('player-progress-line')).toBeHidden();
+  });
+});