Refactor: Migrar UI completa a Tailwind CSS v4 + shadcn-vue

- Reemplazar CSS nativo con Tailwind CSS v4 y utilidades custom
- Crear librería de componentes UI basada en shadcn-vue (Radix Vue)
- Componentes UI: Button, Card, Input, Textarea, Badge, Dialog, Avatar, DropdownMenu
- Migrar todos los componentes existentes a Tailwind utilities
- Convertir EventCard.js (htm) a EventCard.vue (SFC)
- Implementar sistema de temas dark/light con clase .dark
- Mantener efectos glassmorphism via @utility custom
- Eliminar styles.css legacy

frontend/src/components/ui/card/Card.vue

@@ -0,0 +1,27 @@
+<script setup>
+import { computed } from 'vue';
+import { cn } from '@/lib/utils';
+
+const props = defineProps({
+  class: String,
+  variant: {
+    type: String,
+    default: 'default',
+    validator: (v) => ['default', 'glass', 'panel'].includes(v)
+  }
+});
+
+const variantClasses = {
+  default: 'glass-card shadow-md hover:shadow-lg transition-shadow',
+  glass: 'glass glass-border rounded-lg',
+  panel: 'glass-panel border border-pink-200 dark:border-pink-600/50'
+};
+
+const classes = computed(() => cn(variantClasses[props.variant], props.class));
+</script>
+
+<template>
+  <div :class="classes">
+    <slot />
+  </div>
+</template>