Extracted from #39. Replace the all-or-nothing view swap (SessionList replaces ChatView) with a persistent chat plus a right-side history pane: animated open/close, drag-to-resize, overlay fallback when the editor column is narrow, and browse-while-chatting (selecting a session no longer auto-closes the pane).
Implementation notes: consider CSS transitions instead of #39's ~150 lines of rAF animation and manual resize state; add a keyboard/Escape path to dismiss the overlay (missing in #39).
Size: M | Depends on: nothing | Tier: 2
Extracted from #39. Replace the all-or-nothing view swap (SessionList replaces ChatView) with a persistent chat plus a right-side history pane: animated open/close, drag-to-resize, overlay fallback when the editor column is narrow, and browse-while-chatting (selecting a session no longer auto-closes the pane).
Implementation notes: consider CSS transitions instead of #39's ~150 lines of rAF animation and manual resize state; add a keyboard/Escape path to dismiss the overlay (missing in #39).
Size: M | Depends on: nothing | Tier: 2