Improve auth performance: cache profile, remove double fetch, reduce timeout

- Seed currentUser from localStorage instantly on load (no loading flash)
- Remove getSession() duplicate — onAuthStateChange handles everything
- Fetch fresh profile in background while cached version shows immediately
- Reduce fallback timeout from 5s to 2s
- Explicit Supabase client auth options for reliability

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/supabase.js

@@ -5,4 +5,10 @@ const key = import.meta.env.VITE_SUPABASE_ANON_KEY;
 
 if (!url || !key) throw new Error('Missing Supabase environment variables. Check VITE_SUPABASE_URL and VITE_SUPABASE_ANON_KEY.');
 
-export const supabase = createClient(url, key);
+export const supabase = createClient(url, key, {
+  auth: {
+    persistSession: true,
+    autoRefreshToken: true,
+    detectSessionInUrl: false,
+  },
+});