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/search-results.html

@@ -1,4 +1,4 @@
-<div style="background:var(--b2);border:1px solid var(--b3);border-radius:0.5rem;box-shadow:0 8px 24px rgba(0,0,0,0.2);margin-top:0.25rem;max-height:24rem;overflow-y:auto">
+<div style="background:color-mix(in srgb,var(--b1),black 20%);border:1px solid var(--b3);border-radius:0.5rem;box-shadow:0 8px 24px rgba(0,0,0,0.3);margin-top:0.25rem;max-height:24rem;overflow-y:auto">
   {#if artists.isEmpty()}
     <div style="padding:1rem;text-align:center;font-size:0.875rem;color:var(--bc);opacity:0.5">No artists found</div>
   {#else}