fix test: provide fallback supabase URL for unit test env

Prevents createClient from throwing when env vars are missing during
unit tests that transitively import the supabase client.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: francofrizzo

src/lib/supabaseClient.ts

@@ -1,6 +1,6 @@
 import { createClient } from "@supabase/supabase-js";
 
-const supabaseUrl = import.meta.env.VITE_SUPABASE_URL;
-const supabaseAnonKey = import.meta.env.VITE_SUPABASE_ANON_KEY;
+const supabaseUrl = import.meta.env.VITE_SUPABASE_URL ?? "http://localhost";
+const supabaseAnonKey = import.meta.env.VITE_SUPABASE_ANON_KEY ?? "dummy";
 
 export const supabase = createClient(supabaseUrl, supabaseAnonKey);