refactor: extend auth.js types globally (resolves #7)

src/app.d.ts

@@ -1,3 +1,5 @@
+import { DefaultSession } from '@auth/sveltekit';
+
 // See https://svelte.dev/docs/kit/types#app.d.ts
 // for information about these interfaces
 declare global {
@@ -10,4 +12,13 @@ declare global {
 	}
 }
 
+declare module '@auth/sveltekit' {
+	interface Session {
+		user: {
+			id: string;
+			email: string;
+		} & DefaultSession['user'];
+	}
+}
+
 export {};