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/avatar.tsx

@@ -1,69 +1,57 @@
-import * as React from "react"
-import { cva, type VariantProps } from "class-variance-authority"
-import { cn } from "../core/cn"
+import * as React from 'react'
+import { Avatar as MantineAvatar } from '@mantine/core'
 
-const avatarVariants = cva(
-  "relative inline-flex shrink-0 items-center justify-center overflow-hidden rounded-full bg-muted font-medium text-muted-foreground select-none",
-  {
-    variants: {
-      size: {
-        xs: "size-6 text-xs",
-        sm: "size-8 text-sm",
-        md: "size-10 text-base",
-        lg: "size-12 text-lg",
-        xl: "size-16 text-xl",
-      },
-    },
-    defaultVariants: { size: "md" },
-  }
-)
+type AvatarSize = 'xs' | 'sm' | 'md' | 'lg' | 'xl'
 
-interface AvatarProps
-  extends React.ComponentPropsWithoutRef<"span">,
-    VariantProps<typeof avatarVariants> {
+const sizeMap: Record<AvatarSize, string> = {
+  xs: 'sm',
+  sm: 'sm',
+  md: 'md',
+  lg: 'lg',
+  xl: 'xl',
+}
+
+interface AvatarProps extends React.ComponentPropsWithoutRef<'div'> {
   src?: string
   alt?: string
   fallback?: string
   initials?: string
+  size?: AvatarSize
 }
 
 function getInitials(name?: string): string {
-  if (!name) return "?"
+  if (!name) return '?'
   const parts = name.trim().split(/\s+/)
-  if (parts.length === 1) return parts[0].slice(0, 2).toUpperCase()
-  return (parts[0][0] + parts[parts.length - 1][0]).toUpperCase()
+  const first = parts[0] ?? ''
+  const last = parts[parts.length - 1] ?? ''
+  if (parts.length === 1) return first.slice(0, 2).toUpperCase()
+  return ((first[0] ?? '') + (last[0] ?? '')).toUpperCase()
 }
 
-const Avatar = React.forwardRef<HTMLSpanElement, AvatarProps>(
-  ({ className, size, src, alt, fallback, initials, ...props }, ref) => {
-    const [imgError, setImgError] = React.useState(false)
-    const showImage = src && !imgError
+/** Kept for backwards compatibility */
+const avatarVariants = sizeMap
+
+const Avatar = React.forwardRef<HTMLDivElement, AvatarProps>(
+  ({ className, size = 'md', src, alt, fallback, initials, ...props }, ref) => {
     const displayInitials = initials ?? getInitials(fallback ?? alt)
 
     return (
-      <span
+      <MantineAvatar
         ref={ref}
         data-slot="avatar"
-        className={cn(avatarVariants({ size }), className)}
+        src={src}
+        alt={alt ?? ''}
+        size={sizeMap[size]}
+        radius="xl"
+        className={className}
         {...props}
       >
-        {showImage ? (
-          <img
-            src={src}
-            alt={alt ?? ""}
-            className="aspect-square size-full object-cover"
-            onError={() => setImgError(true)}
-          />
-        ) : (
-          <span data-slot="avatar-fallback" aria-hidden="true">
-            {displayInitials}
-          </span>
-        )}
-      </span>
+        {displayInitials}
+      </MantineAvatar>
     )
   }
 )
-Avatar.displayName = "Avatar"
+Avatar.displayName = 'Avatar'
 
 export { Avatar, avatarVariants }
-export type { AvatarProps }
+export type { AvatarProps, AvatarSize }