## Summary

Fixes #3076

`Thread` currently subscribes to `message.read` events to keep its read
state up to date, but never subscribes to `notification.mark_unread`
events. This means that when a user marks a thread as unread, the
`Thread`'s local state never reflects it — `unreadMessageCount` stays
stale until the thread is reloaded.

## Root cause

`Channel` already has a working handler for `notification.mark_unread`
(see `channel.ts`), and `Thread` already has all the state infrastructure
needed (`ThreadUserReadState` with `unreadMessageCount`, `lastReadAt`,
`lastReadMessageId`) — it was just never wired up to this event.

## Changes

- Added `subscribeRepliesMarkUnread()` to `Thread`, following the same
  `.on()` + filter pattern as the existing `subscribeRepliesRead()`
- Registered the new subscription in `registerSubscriptions()`
- Added 3 new test cases in `threads.test.ts`:
  - Ignores events from other threads (filtered by `parent_message_id`)
  - Ignores events for other users (only updates the current user's state)
  - Correctly updates `unreadMessageCount`, `lastReadAt`, and
    `lastReadMessageId` for the current user

## Testing

All existing tests in `threads.test.ts` pass (87/87), including the 3 new
ones added for this fix.

## Notes

- Did not add `first_unread_message_id` tracking since `ThreadUserReadState`
  doesn't currently have an equivalent field (unlike `ChannelState`) —
  happy to add this in a follow-up if desired.
- Did not call `messageReceiptsTracker`, consistent with the existing
  `subscribeRepliesRead()` handler, since thread delivery receipts aren't
  supported yet.