feat: nuevos componentes UI — accordion, avatar, breadcrumb, checkbox, command, dropdown, pagination, popover, radio, sheet, select, switch, textarea, toast

Componentes React accesibles basados en Radix UI con soporte de temas via CSS variables. Incluye barrel export en index.ts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/functions/ui/checkbox.md

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+---
+name: checkbox
+kind: component
+lang: ts
+domain: ui
+version: "1.0.0"
+purity: impure
+signature: "Checkbox(props: CheckboxProps): JSX.Element"
+description: "Input booleano accesible con label opcional y variante indeterminate. Base-UI Checkbox primitive."
+tags: [checkbox, component, ui, interactive, form, base-ui]
+uses_functions: [cn_ts_core]
+uses_types: []
+returns: []
+returns_optional: false
+error_type: ""
+imports: ["@base-ui/react/checkbox", "class-variance-authority"]
+tested: false
+tests: []
+test_file_path: ""
+file_path: "frontend/functions/ui/checkbox.tsx"
+props:
+  - name: label
+    type: "string"
+    required: false
+    description: "Texto de etiqueta visible junto al checkbox"
+  - name: indeterminate
+    type: "boolean"
+    required: false
+    description: "Estado indeterminate (guion) en vez de checked/unchecked"
+  - name: checked
+    type: "boolean"
+    required: false
+    description: "Estado controlado del checkbox"
+  - name: defaultChecked
+    type: "boolean"
+    required: false
+    description: "Estado inicial no controlado"
+  - name: disabled
+    type: "boolean"
+    required: false
+    description: "Deshabilita el checkbox"
+  - name: onCheckedChange
+    type: "(checked: boolean) => void"
+    required: false
+    description: "Callback cuando cambia el estado"
+emits: [onCheckedChange]
+has_state: false
+framework: react
+variant: []
+---
+
+## Ejemplo
+
+```tsx
+// Basico
+<Checkbox label="Acepto los terminos" />
+
+// Controlado
+<Checkbox
+  label="Seleccionar todos"
+  checked={allSelected}
+  indeterminate={someSelected}
+  onCheckedChange={setAllSelected}
+/>
+
+// Sin label
+<Checkbox checked={isActive} onCheckedChange={setIsActive} />
+```
+
+## Notas
+
+Usa Base-UI Checkbox primitive para accesibilidad completa (keyboard, ARIA). El estado indeterminate se muestra con un guion horizontal. El id se genera automaticamente con useId si no se provee.