feat(pwa-offline): Pinia store + IndexedDB; contexto para cache/eliminación; toasts; compatibilidad PWA offline

- Agrega @pinia/nuxt, idb y store central (stores/music.ts)
- Cacheo manual desde menú contextual y borrado (TrackContextMenu)
- Ícono verde para canciones cacheadas, sin auto-cache al reproducir
- Toasts de feedback (stores/toast.ts, ToastContainer)
- Fallback offline de listado a IndexedDB; fix MUSIC_DIR absoluto en preview/prod
- Ajustes PWA: navigateFallback '/', devOptions, workbox condicional
- Estilos y animación del context menu (tema light/dark, blur fuerte)
- Correcciones de sintaxis y posicionamiento exacto al cursor

stores/toast.ts

@@ -0,0 +1,42 @@
+import { defineStore } from 'pinia'
+
+type ToastType = 'success' | 'error' | 'info'
+
+export type Toast = {
+  id: number
+  type: ToastType
+  message: string
+  timeout?: number
+}
+
+let counter = 1
+
+export const useToastStore = defineStore('toast', {
+  state: () => ({
+    toasts: [] as Toast[]
+  }),
+  actions: {
+    push(message: string, type: ToastType = 'info', timeout = 2500) {
+      const id = counter++
+      const toast: Toast = { id, type, message, timeout }
+      this.toasts.push(toast)
+      if (timeout && timeout > 0) {
+        setTimeout(() => this.remove(id), timeout)
+      }
+      return id
+    },
+    success(message: string, timeout = 2500) {
+      return this.push(message, 'success', timeout)
+    },
+    error(message: string, timeout = 3000) {
+      return this.push(message, 'error', timeout)
+    },
+    info(message: string, timeout = 2500) {
+      return this.push(message, 'info', timeout)
+    },
+    remove(id: number) {
+      this.toasts = this.toasts.filter(t => t.id !== id)
+    }
+  }
+})
+