[codex] Fix agent chat preview freeze

[artpi]

Summary

• Replace the SwiftUI multiline composer fields with a fixed-height AppKit text view wrapper so long user input scrolls internally instead of forcing repeated expensive layout measurement.
• Compact long chat messages before Markdown rendering and remove selectable text from chat/table bodies to avoid SwiftUI selection/layout hot paths.
• Reduce preview resize churn by deduping page-state updates, throttling sidebar width updates, and making unchanged chat rows equatable.

Root Cause

The freeze was caused by several SwiftUI hot paths stacking up: long user input in a multiline TextField, very large rendered message bodies, and preview resize/page updates invalidating the chat transcript while scrolling.

Validation

• make CODESIGN_IDENTITY=-
• git diff --check
• Relaunched build/WP Workspace Dev.app and confirmed the process returned to idle after launch.