feat(*): Add basic site functionality including sign up, sign in and sign out

src/lib/stores/user.ts

@@ -0,0 +1,5 @@
+import { writable } from 'svelte/store';
+import { type User } from '@supabase/auth-js';
+
+// Create a user store with null as the initial value
+export const user = writable<User | null>(null);