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/dropzone.tsx

@@ -0,0 +1,20 @@
+import { Dropzone as MantineDropzone, IMAGE_MIME_TYPE, MIME_TYPES } from '@mantine/dropzone'
+import type { DropzoneProps as MantineDropzoneProps } from '@mantine/dropzone'
+import '@mantine/dropzone/styles.css'
+
+interface DropzoneProps extends Partial<MantineDropzoneProps> {
+  children: React.ReactNode
+}
+
+function Dropzone({ children, ...props }: DropzoneProps) {
+  return <MantineDropzone onDrop={() => {}} {...props}>{children}</MantineDropzone>
+}
+
+// Re-export sub-components and constants
+const DropzoneAccept = MantineDropzone.Accept
+const DropzoneReject = MantineDropzone.Reject
+const DropzoneIdle = MantineDropzone.Idle
+const DropzoneFullScreen = MantineDropzone.FullScreen
+
+export { Dropzone, DropzoneAccept, DropzoneReject, DropzoneIdle, DropzoneFullScreen, IMAGE_MIME_TYPE, MIME_TYPES }
+export type { DropzoneProps }