feat: add favicon and remove logo background

Replace default Next.js favicon with the Blackheart logo. Remove white
background from logo PNG for proper transparency in dark mode. Add
unoptimized prop to next/image to preserve alpha channel.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/layout/Header.tsx

@@ -19,6 +19,7 @@ export function Header() {
             alt={BRAND.name}
             width={32}
             height={32}
+            unoptimized
             className="dark:invert"
           />
           <span className="font-display text-lg font-bold tracking-tight">