[Douglasymlai]

Related Issue

Follows the ChatBox timeline work split out of #1958.

Description

Problem

The new conversation timeline (model switch dividers, the header menu, grouped user messages, inline approvals and the narrative task log) is merged, but it only appears in builds that set VITE_CHATBOX_EVENT_BUS=true. No checked-in env file sets it, so every normal build, including the installed app, still shows the old conversation view.

Cause

isChatEventTimelineEnabled() only returned true for the exact value 'true', so an unset variable meant the legacy renderer.

Fix

• Default: the event-native timeline is now on unless a build sets VITE_CHATBOX_EVENT_BUS=false. No env file changes are needed.
• Rollback: building with VITE_CHATBOX_EVENT_BUS=false restores the legacy conversation view. The value is read at build time, so a rollback is a new build. Sessions whose history has no timeline events yet still fall back to the legacy view, as before.
• CI: the Test workflow runs the ChatBox and chat store tests a second time with VITE_CHATBOX_EVENT_BUS=false, so the rollback view stays working while both exist.
• Tests:
  • The task-timer tests for the legacy work log used an empty value to mean "legacy view". They now set false, the rollback value.
  • New tests cover how the flag is read (unset, empty and true mean on; false means off).
• Docs: the ChatBox README describes the new default and the rollback switch.

Testing Evidence

Automated:

• ChatBox, chat store and projection tests: 992 passed (44 files), both with the default and with VITE_CHATBOX_EVENT_BUS=false.
• Full Vitest suite: no new failures compared with main.
• npm run type-check, npm run check:design-tokens, ESLint and Prettier on changed files, and git diff --check: pass.

In the app: not checked yet. Before this is marked ready, a normal dev build (no env changes) should be compared with the old view for:

• new, follow-up and stopped Runs;

• questions, approvals and plan review;

• reopening older Sessions, including ones from before the timeline existed;

• model switches, files and the work log;

• a build with VITE_CHATBOX_EVENT_BUS=false, which should show the old view.

• I have included human-verified testing evidence in this PR.

• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).

• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline.