Sidebar: show each thread's remaining prompt-cache window

[roulendz]

I keep a dozen threads open across projects and I can never tell which ones have
gone cold. I find out by sending a message and watching the whole conversation
get re-read at full price.

The sidebar row already shows how long since a thread last did anything. It just
doesn't say what that costs me.

So I put a small chip in each row, left of the status label. It counts down the
prompt-cache window: 47m, amber under fifteen minutes, red under five, then
cold. Blank if the row has no timestamp to count from. Hover gives last activity
and the clock time it goes cold, in a card styled like the composer's
context-window popover.

Two things I'd fix before this was real. I built it by patching the shipped
bundle, so my version parses the row text. Inside the app it would read
session.status and the same timestamp resolveWorkingStartedAt already uses.

And the window isn't always an hour. It's an hour on a Claude subscription, five
minutes on an API key, and it drops to five minutes on a subscription too once
you start spending usage credits, unless ENABLE_PROMPT_CACHING_1H is set. A
clock also can't tell when the cached prefix stops matching, which is what
switching model, changing effort, /compact or an update actually do. So the
number is a hint, not a promise, and the copy should say so.