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/dashboard_layout.md

@@ -8,12 +8,12 @@ purity: pure
 signature: "dashboardLayout(props: DashboardLayoutProps): ReactElement"
 description: "Genera un grid responsive de dashboard a partir de un array de widgets con span configurable. 1-4 columnas con auto-responsive."
 tags: [dashboard, layout, grid, factory, composition, ui]
-uses_functions: [cn_ts_core]
+uses_functions: []
 uses_types: []
 returns: []
 returns_optional: false
 error_type: ""
-imports: [react]
+imports: [react, "@mantine/core"]
 params:
   - name: props
     desc: "Configuración de layout: número de columnas y array de widgets con id, título, contenido y span"