feat(mejoras): 14 mejoras al CRM Frasa

Fase 1 - UI: dark mode, skeletons, toasts
Fase 2 - Filtros: avanzados Mi día, búsqueda global Ctrl+K
Fase 3 - Datos: export Excel, notas apartados, auditoría
Fase 4 - Dashboard: gráficas vendedor + métricas uso
Fase 5 - Responsive: tablets optimizadas
Fase 6 - PWA: IndexedDB offline, push notifications

Backend: 3 tablas nuevas, 6 services, 6 endpoints nuevos
721 API tests + fronts verdes.

apps/alta-clientes/src/components/DarkModeToggle.tsx

@@ -0,0 +1,32 @@
+import { Sun, Moon } from "lucide-react"
+import { useEffect, useState } from "react"
+
+function getStoredTheme(): boolean {
+  if (typeof window === "undefined") return false
+  try {
+    const stored = localStorage.getItem("frasa_theme")
+    if (stored) return stored === "dark"
+    return window.matchMedia?.("(prefers-color-scheme: dark)").matches ?? false
+  } catch {
+    return false
+  }
+}
+
+export default function DarkModeToggle() {
+  const [dark, setDark] = useState(getStoredTheme)
+
+  useEffect(() => {
+    document.documentElement.classList.toggle("dark", dark)
+    try {
+      localStorage.setItem("frasa_theme", dark ? "dark" : "light")
+    } catch {
+      // localStorage not available (test env)
+    }
+  }, [dark])
+
+  return (
+    <button onClick={() => setDark(!dark)} className="rounded-lg p-2 text-[#64748B] hover:bg-[#F1F5F9] dark:text-[#94A3B8] dark:hover:bg-[#1E293B]" aria-label="Cambiar tema">
+      {dark ? <Sun className="h-5 w-5" /> : <Moon className="h-5 w-5" />}
+    </button>
+  )
+}