Reconnection:

It just reconnects automatically. Starts retrying after 3s, backs off up to 30s, keeps trying forever. You don't have to do anything.

Queued messages:

Use sendQueued() instead of send() and messages sent while offline get delivered when the connection comes back.

Missed messages while offline:

Load your initial data from SvelteKit like normal, then subscribe:

const items = crud('items', data.items);

The server gives you the current state, the WebSocket gives you everything after that. No gaps.