client: add ability to publish (save) posts in browser storage (indexed db?)

[MaxLeiter]

Someone mentioned this on HN and I can see a valid use-case as long as it's made clear that the post will be available only in that browser + never sent to the server + could be removed by the browser at any time.

We should try to use https://developer.mozilla.org/en-US/docs/Web/API/StorageManager and fall-back to indexedb if it's unavailable.