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/simple_select.tsx

@@ -1,8 +1,4 @@
-"use client"
-
-import * as React from "react"
-import { cn } from "../core/cn"
-import { ChevronDown } from "lucide-react"
+import { NativeSelect } from '@mantine/core'
 
 export interface SimpleSelectOption {
   value: string
@@ -23,69 +19,55 @@ interface SimpleSelectProps {
   options: SimpleSelectOptions
   placeholder?: string
   disabled?: boolean
-  size?: "sm" | "default"
+  size?: 'sm' | 'default'
   className?: string
 }
 
 function isGrouped(
   options: SimpleSelectOptions,
 ): options is SimpleSelectGroup[] {
-  return options.length > 0 && "group" in options[0]
+  return options.length > 0 && options[0] != null && 'group' in options[0]
+}
+
+function toMantineData(options: SimpleSelectOptions, placeholder?: string) {
+  const data: Array<{ value: string; label: string; disabled?: boolean } | { group: string; items: Array<{ value: string; label: string; disabled?: boolean }> }> = []
+
+  if (placeholder) {
+    data.push({ value: '', label: placeholder, disabled: true })
+  }
+
+  if (isGrouped(options)) {
+    for (const g of options) {
+      data.push({ group: g.group, items: g.items })
+    }
+  } else {
+    for (const item of options) {
+      data.push(item)
+    }
+  }
+
+  return data
 }
 
 function SimpleSelect({
   value,
   onValueChange,
   options,
-  placeholder = "Select...",
+  placeholder = 'Select...',
   disabled = false,
-  size = "default",
+  size = 'default',
   className,
 }: SimpleSelectProps) {
   return (
-    <div className={cn("relative", className)}>
-      <select
-        value={value}
-        onChange={(e) => onValueChange(e.target.value)}
-        disabled={disabled}
-        className={cn(
-          "flex w-full appearance-none items-center rounded-lg border border-input bg-transparent px-2.5 pr-8 text-sm transition-colors outline-none",
-          "focus-visible:border-ring focus-visible:ring-3 focus-visible:ring-ring/50",
-          "disabled:pointer-events-none disabled:cursor-not-allowed disabled:opacity-50",
-          size === "sm" ? "h-7 text-xs px-2" : "h-8 py-1",
-        )}
-      >
-        {placeholder && !value && (
-          <option value="" disabled>
-            {placeholder}
-          </option>
-        )}
-        {isGrouped(options)
-          ? options.map((g) => (
-              <optgroup key={g.group} label={g.group}>
-                {g.items.map((item) => (
-                  <option
-                    key={item.value}
-                    value={item.value}
-                    disabled={item.disabled}
-                  >
-                    {item.label}
-                  </option>
-                ))}
-              </optgroup>
-            ))
-          : (options as SimpleSelectOption[]).map((item) => (
-              <option
-                key={item.value}
-                value={item.value}
-                disabled={item.disabled}
-              >
-                {item.label}
-              </option>
-            ))}
-      </select>
-      <ChevronDown className="pointer-events-none absolute right-2 top-1/2 size-4 -translate-y-1/2 text-muted-foreground" />
-    </div>
+    <NativeSelect
+      value={value}
+      onChange={(e) => onValueChange(e.currentTarget.value)}
+      data={toMantineData(options, !value ? placeholder : undefined)}
+      disabled={disabled}
+      size={size === 'sm' ? 'xs' : 'sm'}
+      className={className}
+      styles={{ input: { cursor: 'pointer' } }}
+    />
   )
 }