- Move extractText and extractThinking from useGateway and useSecondarySession into shared lib
- Add comprehensive unit tests (13 tests) for both functions
- Eliminate code duplication between the two hooks
- User messages appear instantly with 'sending' state (dimmed, clock icon)
- Transitions to 'sent' (checkmark) when server acknowledges
- Shows error state (alert icon, retry visible) if send fails
- Applied to both primary and secondary sessions
- Add split view button (columns icon) in sidebar session actions
- Click to open any session in a secondary pane alongside the primary
- Resizable divider between panes (drag to resize, persisted in localStorage)
- Secondary pane supports full chat: history, streaming, send, abort
- Close split view via X button or clicking the split icon again
- Each pane has independent scroll, search, and tool collapse
- Keyboard shortcut and i18n support (EN/FR)
