Preview 4 (1d93adb)

fix(terminal): tell Rex the pane's colours

Rax never sent its colours to Rex, so Rex told programs that asked that
the terminal used its own colours and a dark scheme. With a light theme,
programs such as Codex and Neovim picked dark colours. Rax now sends the
active terminal colours and the light or dark scheme with set_theme when
it opens a pane, on each new data connection, and when the theme
changes.