Use color-mix for clearly distinct background tones

- Body: var(--b1) (baseline)
- Cards: color-mix(var(--b1) + 8% white) = lighter surface
- Navbar: color-mix(var(--b1) + 20% black) = darker header
- Cards hover: same as navbar
- Search dropdown: matches navbar darkness

src/main/resources/templates/fragments/artist-list.html

@@ -1,6 +1,6 @@
 <div id="artist-list" hx-swap-oob="true">
 {#if artists.isEmpty()}
-<div style="background:var(--b2);border:1px solid var(--b3);border-radius:0.5rem;padding:2.5rem;text-align:center">
+<div style="background:color-mix(in srgb,var(--b1),white 8%);border:1px solid var(--b3);border-radius:0.5rem;padding:2.5rem;text-align:center">
   <svg class="icon-lg" style="margin:0 auto 0.75rem;color:var(--bc);opacity:0.15" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round">
     <path d="M12 2l3.09 6.26L22 9.27l-5 4.87 1.18 6.88L12 17.77l-6.18 3.25L7 14.14 2 9.27l6.91-1.01L12 2z"/>
   </svg>