fix(seo): generate robots.txt from Astro.site at build

Replace the hardcoded public/robots.txt with src/pages/robots.txt.ts so the Sitemap URL always matches the configured site (no drift between the file and astro.config.mjs).

public/robots.txt

@@ -1,4 +0,0 @@
-User-agent: *
-Allow: /
-
-Sitemap: https://www.eivissaenvironmentalwatch.es/sitemap-index.xml

src/pages/robots.txt.ts

@@ -0,0 +1,18 @@
+import type { APIContext } from 'astro';
+
+export function GET(context: APIContext) {
+  const site = (context.site ?? new URL(context.url.origin)).toString().replace(/\/$/, '');
+  const body = [
+    'User-agent: *',
+    'Allow: /',
+    '',
+    `Sitemap: ${site}/sitemap-index.xml`,
+    '',
+  ].join('\n');
+
+  return new Response(body, {
+    headers: {
+      'Content-Type': 'text/plain; charset=utf-8',
+    },
+  });
+}