feat(header): borderless icon buttons, link-style hover, add Viewer nav

Drop border and card background from header icon buttons (theme, Discord,
GitHub, mobile menu); hover now shifts icon color to primary, matching the
text nav links instead of a gray fill. Add Viewer nav item linking to
viewer.opentaint.org.

Author: misonijnik

src/components/astro/SiteHeader.astro

@@ -3,6 +3,7 @@ const navItems = [
   { href: "/", label: "Home" },
   { href: "/blog", label: "Blog" },
   { href: "https://github.com/seqra/opentaint/blob/main/docs/README.md", label: "Docs", external: true },
+  { href: "https://viewer.opentaint.org/", label: "Viewer", external: true },
 ];
 ---
 

src/index.css

@@ -150,15 +150,12 @@
   }
 
   .theme-button {
-    @apply inline-flex h-11 w-11 items-center justify-center rounded border transition-colors;
-    border-color: hsl(var(--border));
-    background-color: hsl(var(--card));
-    color: hsl(var(--foreground));
+    @apply inline-flex h-11 w-11 items-center justify-center rounded transition-colors;
+    color: hsl(var(--muted-foreground));
   }
 
   .theme-button:hover {
-    border-color: hsl(var(--border-strong));
-    background-color: hsl(var(--secondary));
+    color: hsl(var(--primary));
   }
 
   .theme-icon-sun,