feat: generate PNG favicons and apple-touch-icon from SVG

Adds icon-16/32/48/96/180/192/512.png and favicon.ico (multi-size).
Updates site.webmanifest with 192/512 PNG entries and head.html with
apple-touch-icon and explicit PNG/ICO fallback links.

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

layouts/partials/head.html

@@ -103,5 +103,7 @@
 
 <!-- Favicon & PWA -->
 <link rel="icon" href="/favicon.svg" type="image/svg+xml" />
-<link rel="icon" href="/favicon.ico" sizes="any" />
+<link rel="icon" href="/favicon.ico" sizes="16x16 32x32 48x48" type="image/x-icon" />
+<link rel="icon" href="/icon-96.png" type="image/png" sizes="96x96" />
+<link rel="apple-touch-icon" href="/icon-180.png" sizes="180x180" />
 <link rel="manifest" href="/site.webmanifest" />