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>

static/site.webmanifest

@@ -3,6 +3,8 @@
   "short_name": "Bar Pivoine",
   "description": "A field guide to the good pour.",
   "icons": [
+    { "src": "/icon-192.png", "type": "image/png", "sizes": "192x192" },
+    { "src": "/icon-512.png", "type": "image/png", "sizes": "512x512" },
     { "src": "/favicon.svg", "type": "image/svg+xml", "sizes": "any" }
   ],
   "start_url": "/",