refactor: expose Comlink API via MessagePort in SharedWorker

Update the worker to use the 'onconnect' event to handle incoming
MessagePorts, ensuring each connecting tab receives its own port.

src/workers/journal.worker.ts

@@ -520,4 +520,9 @@ const api = {
 
 export type JournalWorkerAPI = typeof api;
 
-Comlink.expose(api);
+// SharedWorkers communicate via MessagePort from the 'connect' event,
+// not via self.onmessage. Each connecting tab gets its own port.
+(self as unknown as { onconnect: ((e: MessageEvent) => void) | null }).onconnect = (e: MessageEvent) => {
+  const port = e.ports[0];
+  Comlink.expose(api, port);
+};