feat(web): restyle Artists, Discover, Wanted, discography + preview (Phase 2)

Convert the remaining pages onto the Pressing Plant system: PageHead
component, shared list-row / toggle / small+ghost button / tracklist classes,
and page wrappers trimmed now that the shell provides masthead + nav. All
logic and test hooks (aria-labels, button text) preserved. Verified in the
real app across both themes (Playwright): artists/discover/wanted/discography
render cohesively with real data.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/_ui/page-head.tsx

@@ -0,0 +1,8 @@
+export function PageHead({ title, eyebrow }: { title: string; eyebrow?: string }) {
+  return (
+    <div className="page-head">
+      {eyebrow ? <p className="eyebrow">{eyebrow}</p> : null}
+      <h1>{title}</h1>
+    </div>
+  );
+}