Add back the context-left percentage to the TUI status line

The previous context-left percentage was the most useful status indicator in the TUI for me.

It served two practical purposes:

• It showed when a chat was approaching compaction, so I could start a new chat instead of sending more messages to one that was already near the limit.
• It gave me a rough signal for long-context quality degradation. With GPT-5.4, output quality drops heavily in large context scenarios, especially as the conversation went above 272K tokens, and the percentage made that easy to track.

The new bar is maybe visually nicer for some users, but it removes information that is operationally useful for power users.

Please add the context-left percentage back as an optional status-line feature. I am not asking for the new bar to be removed; I am asking for an opt-in way to restore the old signal.