Support automatic pause at end of replay

[benhall-7]

When you reach the end of a replay, the playback should automatically pause on the last frame to allow the user to enter write mode and pick up recording inputs where they last left off.

[benhall-7]

Idea: if the emulator is in read-mode while playing a replay, maybe it should refuse to play beyond the existing inputs. For example, if a user is in write-mode and recording, then they accidentally switch to read-mode, it won't emulate any more frames (and emits a warning message?).