Add Inicio dashboard as homepage

Stat cards (gasto del ciclo, próximas cuotas Tasa Cero, pensión),
ask-the-assistant box that lands in Asistente and auto-sends via
agent.addMessage + runAgent, and últimas transacciones. Index route,
login redirects updated; Asistente stays at /asistente.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/pages/Login.tsx

@@ -23,7 +23,7 @@ export default function LoginPage() {
     try {
       await login(username, password);
       setAuthenticated(true);
-      navigate("/asistente", { replace: true });
+      navigate("/", { replace: true });
     } catch {
       setError("Invalid credentials");
     } finally {