fix(web): keep thread annotation typing responsive

[lastobelus]

The thread annotation editor re-rendered its complete dialog subtree on every keystroke and continuously recalculated the textarea's intrinsic height. This made typing noticeably laggier than the composer.

Keep the draft in the native textarea, update React state only when Save eligibility changes, and use a fixed-height internally scrolling editor. In an integrated browser check, 2,000 input events completed in about 73 ms with one expected UI mutation; the textarea remained 208 px tall.

Validation:

• focused annotation tests
• web typecheck
• targeted lint and formatting
• Quick local CI
• integrated web-client input measurement

Implemented with GPT-5.6-Sol through the Codex harness.