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/textarea.tsx

@@ -1,47 +1,26 @@
-import * as React from "react"
-import { cn } from "../core/cn"
+import * as React from 'react'
+import { Textarea as MantineTextarea } from '@mantine/core'
 
-interface TextareaProps extends React.ComponentPropsWithoutRef<"textarea"> {
+interface TextareaProps extends Omit<React.ComponentProps<typeof MantineTextarea>, 'autosize'> {
   autoResize?: boolean
 }
 
 const Textarea = React.forwardRef<HTMLTextAreaElement, TextareaProps>(
-  ({ className, autoResize = false, onChange, ...props }, ref) => {
-    const internalRef = React.useRef<HTMLTextAreaElement>(null)
-    const resolvedRef = (ref as React.RefObject<HTMLTextAreaElement>) ?? internalRef
-
-    const handleChange = React.useCallback(
-      (e: React.ChangeEvent<HTMLTextAreaElement>) => {
-        if (autoResize && resolvedRef.current) {
-          resolvedRef.current.style.height = "auto"
-          resolvedRef.current.style.height = `${resolvedRef.current.scrollHeight}px`
-        }
-        onChange?.(e)
-      },
-      [autoResize, onChange, resolvedRef]
-    )
-
+  ({ className, autoResize = false, ...props }, ref) => {
     return (
-      <textarea
-        ref={resolvedRef}
+      <MantineTextarea
+        ref={ref}
         data-slot="textarea"
-        className={cn(
-          "min-h-[80px] w-full rounded-lg border border-input bg-transparent px-3 py-2 text-sm transition-colors outline-none",
-          "placeholder:text-muted-foreground",
-          "focus-visible:border-ring focus-visible:ring-3 focus-visible:ring-ring/50",
-          "disabled:pointer-events-none disabled:cursor-not-allowed disabled:bg-input/50 disabled:opacity-50",
-          "aria-invalid:border-destructive aria-invalid:ring-3 aria-invalid:ring-destructive/20",
-          "dark:bg-input/30 dark:disabled:bg-input/80 dark:aria-invalid:border-destructive/50 dark:aria-invalid:ring-destructive/40",
-          autoResize && "resize-none overflow-hidden",
-          className
-        )}
-        onChange={handleChange}
+        autosize={autoResize}
+        minRows={autoResize ? 2 : undefined}
+        size="sm"
+        className={className}
         {...props}
       />
     )
   }
 )
-Textarea.displayName = "Textarea"
+Textarea.displayName = 'Textarea'
 
 export { Textarea }
 export type { TextareaProps }