fix(a11y): make the library badge a badge, not an inert button

`library-status-indicator` was a <button> whose click handler was a
stopPropagation() and a comment saying to wire up the download client
later. On an Explore results page that is 20 of 66 tab stops (measured
in the running app, before and after: 66/20 → 46/0) that announce
themselves as buttons and do nothing.

It is role="img" with its existing label now, and the label for an
unowned entity says "… is not in your library" rather than "Add … to
library" — the old copy was the button's promise written out. The day
there is a download client to call, the right change is a <button>
*with* a handler, not a handler bolted onto something already shaped
like one.

box-sizing: border-box is explicit because a <button> gets it from the
UA stylesheet and a <span> does not, so the badge grew 36px → 38px.
Caught by the stored screenshot.

e2e/specs/album-actions.spec.ts

@@ -64,6 +64,35 @@ test.describe('playing an album from its page', () => {
       app.locator('explore-album-details').locator('.tracklist-legend'),
     ).toContainText('in your library');
   });
+
+  test('the ticks are badges, not keyboard stops', async ({ app }) => {
+    // Every one of them was a <button> whose click handler was a
+    // stopPropagation() and a comment saying to wire up the download
+    // client later: on an Explore results page, 20 of 66 tab stops
+    // promised an action and performed none. Counted here rather than
+    // reasoned about, because the count is the finding.
+    const stops = await app.evaluate(() => {
+      const badges = [
+        ...(document
+          .querySelector('explore-album-details')
+          ?.shadowRoot?.querySelectorAll('library-status-indicator') ?? []),
+      ];
+
+      return {
+        badges: badges.length,
+        focusable: badges.filter((b) =>
+          b.shadowRoot?.querySelector('button, [tabindex]:not([tabindex="-1"])'),
+        ).length,
+        labelled: badges.filter((b) =>
+          b.shadowRoot?.querySelector('[role="img"][aria-label]'),
+        ).length,
+      };
+    });
+
+    expect(stops.badges).toBeGreaterThan(0);
+    expect(stops.focusable).toBe(0);
+    expect(stops.labelled).toBe(stops.badges);
+  });
 });
 
 /** Albums → click the second card, which navigates to the album page. */