[Feature]: Add PlaybackControls for live spectators

[itsalaidbacklife]

Feature Summary

MUST DECIDE WHETHER TO DO THIS AFTER TESTING: We should allow players to navigate back to previous moves (and forward again) using the PlaybackControls while they live spectate a game

Detailed Description

As of #1203, users can rewatch previously finished games and use the PlaybackControls to navigate between game states. The app uses the gameHistoryStore's showPlaybackControls computed property to decide whether to show these, and currently that returns true if and only if:

1. We're spectating a game
2. That game is not ongoing

So live games can be spectated but always by watching the current move. It could be nice to be able to use the playback controls to navigate backwards to a previous move while watching a game live. To do that we could update the above computed property to lift the restriction of only using the playback controls during finished games.

We should then add test cases for using the PlaybackControls during live games to ensure that:

1. You can move back and forth to previous game states
2. When new socket events come in, you navigate directly to the latest game state