Fix chat composer lag in long threads

[Yuqing (mydmdm)]

Summary

• isolate the thread-local draft subscription inside a dedicated composer boundary so typing does not rerender ChatPanel or message history
• memoize the message list and unchanged user, assistant, and Milkdown message cards while preserving streaming updates
• cache textarea resize style metrics and avoid redundant style writes
• add direct-input and Chinese IME regression coverage plus a real-browser 0-vs-200-message input-to-paint benchmark
• document the composer/history render boundary

Validation

• pnpm format
• pnpm lint:fix
• pnpm typecheck
• targeted Vitest suite: 7 passed
• Playwright composer benchmark: direct and Chinese IME p95 ~16.8 ms for both 0 and 200 messages; history delta ~0 ms

Fixes #115