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/dropdown_menu.md

@@ -7,13 +7,13 @@ version: "1.0.0"
 purity: impure
 signature: "DropdownMenu(props: DropdownMenuProps): JSX.Element"
 description: "Menu de acciones y contexto accesible con items, checkboxes, radios, separadores y submenus. Base-UI Menu primitive."
-tags: [dropdown, menu, component, ui, interactive, overlay, base-ui]
-uses_functions: [cn_ts_core]
+tags: [dropdown, menu, component, ui, interactive, overlay, mantine]
+uses_functions: []
 uses_types: []
 returns: []
 returns_optional: false
 error_type: ""
-imports: ["@base-ui/react/menu", "lucide-react"]
+imports: ["@mantine/core"]
 output: "Componente DropdownMenu que renderiza menú desplegable accesible con items, checkboxes, radios y submenus"
 tested: false
 tests: []