chore: add fallback tier to sponsorkit config

sponsorkit.config.ts

@@ -13,6 +13,10 @@ export default defineConfig({
       monthlyDollars: -1,
       preset: tierPresets.xs,
     },
+    {
+      title: 'Sponsors',
+      preset: tierPresets.base,
+    },
     {
       title: 'Explorers',
       monthlyDollars: 5,