fix(ui): open one session accordion panel at a time

PROJECT_STATE.md

@@ -1,6 +1,6 @@
 # ThothII — Project State
 
-Last updated: 2026-09-13.
+Last updated: 2026-09-14.
 
 This file is the short operational snapshot. Stable commands and the architecture mental model
 live in `AGENTS.md`; current design and runtime contracts live under `docs/architecture/`,
@@ -50,18 +50,19 @@ The backend readiness gate is unchanged.
 Active sessions (non-archived) and Archive both start collapsed. Their adjacent
 headers are the only visible content below the scope tabs: no Sessions heading
 or external selection toolbar. Each nonempty panel owns its Select all and
-bulk-delete controls, scoped to that list and preserving the other's selection. Both
-accordion sections can remain open and share the remaining sidebar height,
-with independently scrolling content capped at `min(18rem, 35dvh)`. The mobile
+bulk-delete controls, scoped to that list and preserving the other's selection.
+As of 2026-09-14, only one section can be open at a time, and either can be
+collapsed. Empty lists show only "No sessions yet." The open section uses the
+remaining sidebar height, with scrolling content capped at `min(18rem, 35dvh)`. The mobile
 navigation dialog also provides a bounded height. Keyboard controls and labels
 are retained. See `DESIGN.md` and `docs/guida-utente.md` for the UI contract.
 
-Verification: 766 frontend unit tests, 20 browser scenarios (including 80 mocked
+Verification: 768 frontend unit tests, 20 browser scenarios (including 80 mocked
 sessions at 390/1280px), TypeScript and the production frontend build passed.
 Only the Mac frontend was recreated; it is healthy at `127.0.0.1:8080`.
 Core, catalog, Qdrant and embedding containers were not changed. The prior
 frontend image is retained as
-`thothii-frontend:before-scoped-session-selection-20260913` for rollback.
+`thothii-frontend:before-single-session-accordion-20260914` for rollback.
 
 ### Agreed Omics delivery route — 2026-09-13