chore: Add Zeabur configuration

* .node-version has been degraded to v20, since
  Svelte does not support v22 on Serverless :(
* Add Zeabur adapter (only used when ZEABUR=1)

.node-version

@@ -1 +1 @@
-v22.6.0
+v20

package.json

@@ -36,6 +36,7 @@
 	},
 	"type": "module",
 	"dependencies": {
+		"@zeabur/svelte-adapter": "^1.0.0",
 		"svelte-boring-avatars": "^1.2.6",
 		"tailwind-merge": "^2.5.2",
 		"typewriter-editor": "^0.12.6",

svelte.config.js

@@ -1,4 +1,5 @@
 import adapter from '@sveltejs/adapter-auto';
+import zAdapter from '@zeabur/svelte-adapter';
 import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
 
 /** @type {import('@sveltejs/kit').Config} */
@@ -11,7 +12,7 @@ const config = {
 		// adapter-auto only supports some environments, see https://kit.svelte.dev/docs/adapter-auto for a list.
 		// If your environment is not supported, or you settled on a specific environment, switch out the adapter.
 		// See https://kit.svelte.dev/docs/adapters for more information about adapters.
-		adapter: adapter()
+		adapter: process.env.ZEABUR ? zAdapter() : adapter()
 	}
 };