fix(chat): preserve browser-owned IME composition

[nwparker]

Summary

• leave active textarea composition owned by Chromium instead of driving it with a controlled React value
• synchronize external drafts only while the browser is idle and reconcile the live DOM value at composition end
• stop IME-owned events at the existing application-action boundary for submit, picker, typing redirect, font scale, and chat toggle
• remove the duplicate parent composition ref and callback plumbing

NativeChatComposerField already existed. This adds no class, component abstraction, tracker module, registry, or production file.

Native evidence

A physical macOS 2-Set Korean Electron journey types 테스트 into session chat while the agent is working:

• the same textarea retains 테스트 through 1.5 seconds of streaming rerenders
• the recorded Enter shape is key Enter, keyCode 229, isComposing true
• compositionstart and compositionend are both present in the attached Playwright trace
• exact PTY input is 테스트 followed by newline, once
• the paired ordinary path is ordinary followed by newline

Mutation checks

• deleting the single IME action guard duplicates the composed send in the physical journey while the ordinary path remains ordinary
• changing the textarea back to controlled value draft fails only the active-composition positive; all six idle and ordinary field tests pass
• restoring preventDefault on IME Enter does not change the native macOS result; its removal is a native-ownership simplification, not the trace-proven mechanism

Verification

• physical macOS Korean headful Electron E2E: 1 passed
• focused native-chat tests: 24 passed
• changed-file Oxlint and formatting
• node and web typechecks
• max-lines ratchet and diff check

Remaining certification

This remains draft. Windows native Korean evidence, reporter confirmation, and issue-specific classification are still required by the goalpost. Terminal and CLI/TUI reports remain owned by the stacked terminal PR rather than this native-chat path.