chore: add sitemap integration and update favicon

- Add @astrojs/sitemap and configure in astro.config.mjs
- Add favicon.svg, apple-touch-icon.png, icon-192.png, icon-512.png
- Add site.webmanifest and update Layout.astro to reference icons and
  manifest
- Update robots.txt to include sitemap
- Update favicon and icon links in Layout.astro

public/robots.txt

@@ -1,5 +1,4 @@
-User-agent: Googlebot
-Disallow: /nogooglebot/
-
 User-agent: *
-Allow: /
+Allow: /
+
+Sitemap: https://fiatcode.dev/sitemap-xml

public/site.webmanifest

@@ -0,0 +1,11 @@
+{
+  "name": "fiatcode",
+  "short_name": "fiatcode",
+  "icons": [
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png" }
+  ],
+  "theme_color": "#171717",
+  "background_color": "#171717",
+  "display": "standalone"
+}