Add environment variable configuration for WEBMENTION_TOKEN and simplify token handling in fetchMentions function

astro.config.mjs

@@ -2,7 +2,7 @@
 
 import mdx from '@astrojs/mdx';
 import sitemap from '@astrojs/sitemap';
-import { defineConfig, fontProviders } from 'astro/config';
+import { defineConfig, envField, fontProviders } from 'astro/config';
 
 import node from '@astrojs/node';
 
@@ -10,6 +10,16 @@ import node from '@astrojs/node';
 export default defineConfig({
   site: 'https://adrian-altner.de',
 
+  env: {
+    schema: {
+      WEBMENTION_TOKEN: envField.string({
+        context: 'server',
+        access: 'secret',
+        optional: true,
+      }),
+    },
+  },
+
   devToolbar: {
       enabled: false,
 	},