[codex] Refine sidebar and terminal tabs layout

[vaayne]

Summary

• keep the sidebar as one surface with an icon-only collapsed width
• move tmux window switching into a terminal tab strip above the Ghostty surface
• reduce sidebar visual weight with a tighter expanded width and compact worktree rows

Validation

• swift build --product Mori
• swift build -c release --product Mori

Notes

• The two local mock-up files under assets/ remain untracked and are not part of this PR.