feat(api): make ApiClient public and add Store::local for custom backends

[hboisgibault]

Motivation

Store was only constructible from in-crate backends (MemStore, FsStore) or via Store::connect (remote). External crates could not back a Store with a custom implementation (e.g. OPFS in the browser/WASM, in-memory test backend) by serving proto::Request on a local irpc channel, because ApiClient and the local constructor were pub(crate).

Changes

• Make ApiClient (irpc::Client<proto::Request>) public.
• Add Store::local(client: ApiClient) public constructor: create a Store from a custom local rpc client.
• Keep Store::from_sender as pub(crate) thin wrapper over Store::local (no breaking change).

Example

let (client, rx) = irpc::channel::local(...); // serve proto::Request with your backend
tokio::spawn(async move { /* handle rx with custom backend, e.g. OPFS */ });
let store = Store::local(client);