fix(sync): send credentials on cross-origin couchdb requests

PouchDB 8+ ignores the legacy ajax config, so withCredentials had no
effect and the AuthSession cookie was stripped from cross-origin sync
requests. Switch to a fetch wrapper that sets credentials: 'include'
so replication authenticates and pushes local docs to the remote.

src/services/CouchService.ts

@@ -11,11 +11,11 @@ PouchDb.plugin(PouchDbAuthentication)
 
 const emit = debounce((ev: typeof SYNC, arr?: string[]) => bus.emit(ev, arr), 150)
 
-const remoteConfig = {
+const remoteConfig: PouchDB.Configuration.RemoteDatabaseConfiguration = {
   skip_setup: true,
-  ajax: {
-    cache: true,
-    withCredentials: true
+  fetch: (url, opts) => {
+    const init: RequestInit = { ...(opts ?? {}), credentials: 'include' }
+    return PouchDb.fetch(url as RequestInfo, init)
   }
 }
 const LOCALE_DB = 'VAQUANT_LOCALE_DB'