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/date_picker_input.tsx

@@ -0,0 +1,17 @@
+import { DatePickerInput as MantineDatePickerInput, DatePicker as MantineDatePicker } from '@mantine/dates'
+import type { DatePickerInputProps as MantineDatePickerInputProps, DatePickerProps as MantineDatePickerProps } from '@mantine/dates'
+import '@mantine/dates/styles.css'
+
+interface DatePickerInputProps extends MantineDatePickerInputProps<'default'> {}
+interface DatePickerProps extends MantineDatePickerProps<'default'> {}
+
+function DatePickerInput(props: DatePickerInputProps) {
+  return <MantineDatePickerInput {...props} />
+}
+
+function DatePicker(props: DatePickerProps) {
+  return <MantineDatePicker {...props} />
+}
+
+export { DatePickerInput, DatePicker }
+export type { DatePickerInputProps, DatePickerProps }