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/vite.config.ts

@@ -1,13 +1,15 @@
 import { defineConfig } from "vite";
 import react from "@vitejs/plugin-react";
-import tailwindcss from "@tailwindcss/vite";
 import path from "path";
 
 export default defineConfig({
-  plugins: [react(), tailwindcss()],
+  plugins: [react()],
   resolve: {
     alias: {
       "@": path.resolve(__dirname, "./src"),
     },
   },
+  css: {
+    postcss: path.resolve(__dirname, "./postcss.config.cjs"),
+  },
 });