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/label.tsx

@@ -1,14 +1,15 @@
-import * as React from "react"
-import { cn } from "../core/cn"
+import * as React from 'react'
+import { Text } from '@mantine/core'
 
-function Label({ className, ...props }: React.ComponentProps<"label">) {
+function Label({ className, ...props }: React.ComponentProps<'label'>) {
   return (
-    <label
+    <Text
+      component="label"
       data-slot="label"
-      className={cn(
-        "flex items-center gap-2 text-sm leading-none font-medium select-none group-data-[disabled=true]:pointer-events-none group-data-[disabled=true]:opacity-50 peer-disabled:cursor-not-allowed peer-disabled:opacity-50",
-        className
-      )}
+      size="sm"
+      fw={500}
+      style={{ display: 'inline-flex', alignItems: 'center', gap: 8, userSelect: 'none' }}
+      className={className}
       {...props}
     />
   )