Add a "Compact now" action to the context window meter

[Pleko-script]

The context window meter already shows how much context a thread is using and whether the provider compacts automatically.

What is missing is a direct way to manually compact the current thread.

Right now, if I want to compact early, I have to know that /compact exists and type it manually. The context window popover feels like the natural place for this action, especially when it is already warning that the thread is getting close to its limit.

Proposal

Add a small Compact now action to the existing context window popover.

It should use the same provider command path as typing /compact, rather than adding a separate compaction mechanism.

The action should only appear when the active provider supports manual compaction. While compaction is running, it could be disabled and show Compacting....

Something simple like:

Context Window
78% · 156k / 200k

[ Compact now ]

Smallest useful scope

Web/desktop only to start with:

• Add the action to the existing ContextWindowMeter popover.
• Only show it when manual compaction is supported by the active provider.
• Reuse the existing /compact command behavior.
• No new settings.
• No changes to automatic compaction.

Mobile could use the same action later once it has equivalent context-window controls.

Related

#7652 is related but solves a different part of the flow. It proposes showing when compaction is running and whether it succeeded. This proposal is only about giving the user a direct way to start compaction from the context window UI.

The two would work well together: Compact now starts it, and #7652 makes the resulting compaction state visible.