fix(webui): hide assistant actions until turn end

[ZhouJ-sh]

Summary

• hide assistant copy and fork actions until the containing Agent turn receives turn_end
• preserve historical actions using authoritative active turn IDs with a legacy boundary fallback
• handle guidance rows, failed optimistic sends, resumed turns, and assistant-only automation runs

Testing

• NODE_OPTIONS='--localstorage-file=/tmp/nanobot-vitest-fix-review-9' bun run test -- src/tests/thread-messages.test.tsx src/tests/thread-viewport.test.tsx src/tests/thread-shell.test.tsx (133 passed)
• bun run lint
• bun run build
• git diff --check

Closes #5368