[FEATURE] Show resulting context-window % when using esc+esc to rewind the conversation

What

When pressing esc esc to rewind/jump back to an earlier point in the conversation, show the context window percentage you'll land on for each rewind target — ideally as a transition, e.g. 42% → 20%.

Why

One of the main reasons I use esc esc is to reclaim context window space, not just to edit a past message. Today the rewind UI shows the messages you can jump back to, but it gives no indication of how much context each choice frees up. I have to guess which earlier point will get me back under a comfortable threshold.

Surfacing the resulting context % per candidate would let me pick the right rewind target deliberately ("jump back far enough to get from 42% to ~20%") instead of trial-and-error.

Proposed behavior

• In the esc esc rewind selector, annotate each selectable point with the context-window usage you'd be left with after rewinding to it.
• Showing it as a transition (current% → resulting%) makes the savings obvious at a glance.

Notes

This is purely additive UI on an existing feature — the rewind already knows the message it would truncate to, so the resulting token/context count should be computable from the same data.