webui: shrink TUI console font from 13px to 11px

[edwin-zvs]

Summary

• The web UI's full TUI mode ("Open TUI console") used a fixed xterm fontSize of 13.
• Drop it two steps to 11, matching the desktop session-pane terminal density.
• Session terminals and hover previews are unchanged (they already use layout-aware / separate sizes).

Test plan

• Open web UI → Open TUI console
• Confirm the TUI renders denser / smaller text than before
• Resize the overlay / window and confirm fit still works
• Session panes still use their existing font sizes (11 desktop / 13 mobile)