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

### Short summary

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### Affected version or release

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### Installation context

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### 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

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### Expected behavior

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### Additional context

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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.

Written by the indexing model from the issue text.