Show context widget in prompt composer when context reaches >80%

### Short summary

_No response_

### Affected version or release

_No response_

### Installation context

_No response_

### What happened?

In situations where developers like to manually compact, show the context widget when a compaction event is approaching. This approach will keep prompt composer minimal, but still allow control for power users who manually compact.

### Steps to reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_

Start by locating the prompt composer and its context widget, then trace how the app detects an approaching compaction event. Verify that the widget appears when context exceeds 80% while the composer remains minimal below that threshold, including the manual compaction flow.

Rédigé par le modèle d'indexation à partir du texte de l'issue.