More permanent and correct solution to old notifications

[corrideat]

Problem

In #2629, notification sounds were played when joining a chatroom (for old messages). This was addressed by calling sbp('chelonia/contract/isSyncing', contractID, { firstSync: true }) in messageReceivePostEffect.

This works most of the time, but it's not perfect. For example, it will result in those same sounds being played if the initial contract sync gets interrupted.

Solution

A more permanent and correct solution could be as follows:

1. When joining a chatroom, the device takes note of the latest height and stores it. Note: this is a per-device setting, distinct from the information that's part of the contract state. This is only done once (per join, i.e., leaving and re-joining resets this height, but otherwise it doesn't get updated).
2. messageReceivePostEffect checks if the message height is higher than this value before emitting a notification.