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/form_field.tsx

@@ -1,5 +1,5 @@
-import * as React from "react"
-import { cn } from "../core/cn"
+import * as React from 'react'
+import { Box, Text } from '@mantine/core'
 
 interface FormFieldProps {
   label?: string
@@ -15,26 +15,39 @@ function FormField({ label, helperText, error, children, className }: FormFieldP
   const helperId = `${id}-helper`
   const errorId = `${id}-error`
 
-  const describedBy = [helperText ? helperId : null, error ? errorId : null].filter(Boolean).join(" ") || undefined
+  const describedBy = [helperText ? helperId : null, error ? errorId : null].filter(Boolean).join(' ') || undefined
 
   const childWithProps = React.Children.map(children, (child) => {
     if (React.isValidElement(child)) {
       return React.cloneElement(child as React.ReactElement<Record<string, unknown>>, {
         id: inputId,
-        "aria-invalid": error ? true : undefined,
-        "aria-describedby": describedBy,
+        'aria-invalid': error ? true : undefined,
+        'aria-describedby': describedBy,
+        error: error || undefined,
       })
     }
     return child
   })
 
   return (
-    <div className={cn("flex flex-col gap-1.5", className)}>
-      {label && <label htmlFor={inputId} className="text-sm font-medium text-foreground">{label}</label>}
+    <Box className={className}>
+      {label && (
+        <Text component="label" htmlFor={inputId} size="sm" fw={500} mb={4} display="block">
+          {label}
+        </Text>
+      )}
       {childWithProps}
-      {helperText && !error && <p id={helperId} className="text-sm text-muted-foreground">{helperText}</p>}
-      {error && <p id={errorId} className="text-sm text-destructive">{error}</p>}
-    </div>
+      {helperText && !error && (
+        <Text id={helperId} size="sm" c="dimmed" mt={4}>
+          {helperText}
+        </Text>
+      )}
+      {error && (
+        <Text id={errorId} size="sm" c="red" mt={4}>
+          {error}
+        </Text>
+      )}
+    </Box>
   )
 }