refactor: migrar Select y SimpleSelect a native HTML select

Select reescrito de @base-ui/react primitives a <select> nativo con wrapper
para mantener la misma API visual (ChevronDown, estilos tema). SimpleSelect
actualizado para usar <select>/<optgroup> directamente sin intermediarios.
Checkbox corregido: import CheckboxIndicator separado reemplazado por
CheckboxPrimitive.Indicator para consistencia.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/functions/ui/checkbox.tsx

@@ -1,6 +1,5 @@
 import * as React from "react"
 import { Checkbox as CheckboxPrimitive } from "@base-ui/react/checkbox"
-import { CheckboxIndicator } from "@base-ui/react/checkbox"
 import { cn } from "../core/cn"
 
 interface CheckboxProps extends CheckboxPrimitive.Root.Props {
@@ -30,7 +29,7 @@ function Checkbox({ className, label, id, indeterminate, ...props }: CheckboxPro
         indeterminate={indeterminate}
         {...props}
       >
-        <CheckboxIndicator
+        <CheckboxPrimitive.Indicator
           data-slot="checkbox-indicator"
           className="flex items-center justify-center text-current"
         >
@@ -60,7 +59,7 @@ function Checkbox({ className, label, id, indeterminate, ...props }: CheckboxPro
               <polyline points="20 6 9 17 4 12" />
             </svg>
           )}
-        </CheckboxIndicator>
+        </CheckboxPrimitive.Indicator>
       </CheckboxPrimitive.Root>
       {label && (
         <label