feat(explore): arrows on every sideways row, and one album card size

The shelves, the top results and the artist page's discography and
similar-artists rows scrolled sideways only by a horizontal wheel or a
trackpad, so a plain mouse could not reach anything past the fold.
<scroll-row> wraps each of them with previous/next arrows that are
hidden at the end they cannot move from, revealed on hover where there
is hover, and always shown where there is not.

The album card is defined once, in albumCardStyles, instead of twice.
explore-view clamped its cards to 130-150px, and with square artwork
that made cards in one row different heights as well as widths.  The
width is fixed now and each line under the art reserves its own space.
Covers are inset rather than cropped (contain, not cover).

The ownership badge sits over the artwork for owned and unowned alike,
and catalog cards no longer dim: a grid of dimmed covers read as a page
that had failed to load.  The album page's tracklist still dims unowned
rows, which says something different about something different.

Every MusicBrainz link now goes through openMusicBrainz, which pins the
origin, including the one on the album page that still called
window.open directly.  solid/chevron-left is bundled for the left arrow;
without it the arrow drew the missing-icon fallback.

Closes #264

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

frontend/test/components/artist-release-menu.test.ts

@@ -25,7 +25,9 @@ const ARTIST = 'artist-0001';
 
 /** The labels of the open menu's items, trimmed. */
 function menuItems(el: LitElement): string[] {
-  const panel = shadow(el, '.context-menu-panel');
+  // Scoped to the context menu: the artist page also has a Play/Shuffle
+  // dropdown, and its panel carries the same class.
+  const panel = shadow(el, '#context-menu .context-menu-panel');
 
   if (!panel) return [];
 
@@ -100,7 +102,7 @@ describe('the context menu on an artist page release', () => {
 
     await openMenuOnAlbum(el, 0);
 
-    const panel = shadow(el, '.context-menu-panel');
+    const panel = shadow(el, '#context-menu .context-menu-panel');
 
     expect(panel).toBeTruthy();
     // The panel is shared with the track menu, so a label that does not