PartyLayer/PartyLayer: docs/vue-pinia.md

# Pinia integration (optional) `@partylayer/vue` ships an optional Pinia integration for teams that centralize state in Pinia. It is an OPTION, not the default. The default is provide/inject (`provideSessionStore` / `createPartyLayerSession`) plus the composables (`useSession` / `useAccount` / `useP