Add Supabase client and env template

frontend/.env.example

@@ -0,0 +1,3 @@
+VITE_SUPABASE_URL=https://YOURPROJECT.supabase.co
+VITE_SUPABASE_PUBLISHABLE_KEY=sb_publishable_YOURKEY
+VITE_API_URL=http://localhost:5057

frontend/src/lib/supabase.ts

@@ -0,0 +1,12 @@
+import { createClient } from '@supabase/supabase-js';
+
+const url = import.meta.env.VITE_SUPABASE_URL;
+const publishableKey = import.meta.env.VITE_SUPABASE_PUBLISHABLE_KEY;
+
+if (!url || !publishableKey) {
+  throw new Error(
+    'Missing VITE_SUPABASE_URL or VITE_SUPABASE_PUBLISHABLE_KEY. Copy .env.example to .env.local and fill it in.'
+  );
+}
+
+export const supabase = createClient(url, publishableKey);