refactor: migrate frontend from shadcn/Tailwind to Mantine v9

Reescribe todos los componentes UI para usar Mantine v9 en lugar de shadcn/Tailwind.
Elimina cn(), CVA, components.json, theme_provider custom y globals.css con Tailwind.
Añade 25+ componentes nuevos (AppShell, AuthForm, DatePickerInput, Dropzone, etc.)
y MantineProvider como wrapper estándar del sistema de temas.

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

frontend/functions/ui/select.tsx

@@ -1,91 +1,10 @@
-"use client"
+import { Select as MantineSelect, type SelectProps as MantineSelectProps } from '@mantine/core'
 
-import * as React from "react"
-import { cn } from "../core/cn"
-import { ChevronDown } from "lucide-react"
+interface SelectProps extends MantineSelectProps {}
 
-// ── Native select wrapper ─────────────────────────────────────────────────
-
-interface SelectProps extends React.ComponentPropsWithoutRef<"select"> {
-  placeholder?: string
+function Select(props: SelectProps) {
+  return <MantineSelect {...props} />
 }
 
-const Select = React.forwardRef<HTMLSelectElement, SelectProps>(
-  ({ className, children, placeholder, ...props }, ref) => {
-    return (
-      <div className="relative">
-        <select
-          ref={ref}
-          data-slot="select"
-          className={cn(
-            "flex h-8 w-full appearance-none items-center rounded-lg border border-input bg-transparent px-2.5 pr-8 py-1 text-sm transition-colors outline-none",
-            "focus-visible:border-ring focus-visible:ring-3 focus-visible:ring-ring/50",
-            "disabled:pointer-events-none disabled:cursor-not-allowed disabled:opacity-50",
-            "aria-invalid:border-destructive aria-invalid:ring-destructive/20",
-            className,
-          )}
-          {...props}
-        >
-          {placeholder && (
-            <option value="" disabled>
-              {placeholder}
-            </option>
-          )}
-          {children}
-        </select>
-        <ChevronDown className="pointer-events-none absolute right-2 top-1/2 size-4 -translate-y-1/2 text-muted-foreground" />
-      </div>
-    )
-  },
-)
-Select.displayName = "Select"
-
-// ── Sub-components (thin wrappers for API compatibility) ──────────────────
-
-function SelectItem({
-  className,
-  ...props
-}: React.ComponentPropsWithoutRef<"option">) {
-  return <option className={className} {...props} />
-}
-
-function SelectGroup({
-  className,
-  ...props
-}: React.ComponentPropsWithoutRef<"optgroup">) {
-  return <optgroup className={className} {...props} />
-}
-
-function SelectGroupLabel(_props: { children?: React.ReactNode }) {
-  // optgroup uses `label` attr, this is a no-op for compatibility
-  return null
-}
-
-// Stubs for barrel export compatibility — these are no-ops with native select
-function SelectContent({ children }: { children?: React.ReactNode; className?: string }) {
-  return <>{children}</>
-}
-function SelectTrigger({ children }: { children?: React.ReactNode; className?: string }) {
-  return <>{children}</>
-}
-function SelectValue(_props: { placeholder?: string }) {
-  return null
-}
-function SelectPortal({ children }: { children?: React.ReactNode }) {
-  return <>{children}</>
-}
-function SelectSeparator({ className, ...props }: React.ComponentProps<"div">) {
-  return <div className={cn("-mx-1 my-1 h-px bg-muted", className)} {...props} />
-}
-
-export {
-  Select,
-  SelectContent,
-  SelectGroup,
-  SelectGroupLabel,
-  SelectItem,
-  SelectPortal,
-  SelectSeparator,
-  SelectTrigger,
-  SelectValue,
-}
+export { Select }
+export type { SelectProps }