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/multi_select.tsx

@@ -0,0 +1,10 @@
+import { MultiSelect as MantineMultiSelect, type MultiSelectProps as MantineMultiSelectProps } from '@mantine/core'
+
+interface MultiSelectProps extends MantineMultiSelectProps {}
+
+function MultiSelect(props: MultiSelectProps) {
+  return <MantineMultiSelect {...props} />
+}
+
+export { MultiSelect }
+export type { MultiSelectProps }