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/card.md

@@ -8,7 +8,7 @@ purity: impure
 signature: "Card(props: { size?: 'default' | 'sm'; variant?: 'default' | 'borderless' | 'ghost'; className?: string; children: ReactNode }): JSX.Element"
 description: "Contenedor card con header, title, description, action, content y footer. Sistema de slots composable. Variantes default, borderless y ghost para dashboards dark."
 tags: [card, container, layout, component, ui, dashboard, dark]
-uses_functions: [cn_ts_core]
+uses_functions: []
 uses_types: []
 returns: []
 returns_optional: false