feat: add PWA and favicon assets

apple-touch-icon, android-chrome icons (192/512), PNG favicons
(16/32), ICO fallback, and site.webmanifest with heat theme color.

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

static/site.webmanifest

@@ -0,0 +1,19 @@
+{
+  "name": "Pivoine.Art",
+  "short_name": "Pivoine",
+  "icons": [
+    {
+      "src": "/android-chrome-192x192.png",
+      "sizes": "192x192",
+      "type": "image/png"
+    },
+    {
+      "src": "/android-chrome-512x512.png",
+      "sizes": "512x512",
+      "type": "image/png"
+    }
+  ],
+  "theme_color": "#FF1A8C",
+  "background_color": "#050510",
+  "display": "standalone"
+}