Fix: agregar /api/auth/signin a PUBLIC_ROUTES

Sin esto el middleware redirige a /login antes de entrar al handler,
haciendo que el botón "Continuar con Google" cicle contra el propio
login sin llegar nunca a Google.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/middleware.ts

@@ -2,7 +2,7 @@ import { defineMiddleware } from 'astro:middleware';
 import { serverClient } from '@/lib/supabase';
 
 const UABC_DOMAIN = '@uabc.edu.mx';
-const PUBLIC_ROUTES = ['/login', '/api/auth/callback', '/api/auth/signout'];
+const PUBLIC_ROUTES = ['/login', '/api/auth/signin', '/api/auth/callback', '/api/auth/signout'];
 
 export const onRequest = defineMiddleware(async (context, next) => {
   const supabase = serverClient(context.cookies);