feat(flowchat): stabilize timeline and session pane layout

[GCWing]

Summary

• Reconcile FlowChat timeline blocks and reader state across streaming, grouping, search, virtualization, and tool disclosure.
• Add the shared SplitView and synchronize session pane layout and ownership.
• Bound Markdown, code, diff, and operation-summary work; update related theme tokens and Design Lab contracts.
• Add focused unit coverage and native E2E specifications.

Validation

• Design system build and test suite passed.
• Web UI type check passed; lint passed with warnings only.
• i18n, theme color, Appearance, and repository hygiene audits passed.
• Focused Web UI tests passed: 44 files / 559 tests, followed by 10 files / 82 tests after lint fixes.
• git diff --check and merge-tree against origin/main passed.

Changed Markdown design, review, and process files were excluded. The generated API bindings used for local type checking remain ignored. Native visual and remote-host scenarios were not exercised; no browser automation or mock visual validation was used.