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

DESIGN.md

@@ -313,14 +313,16 @@ default, hover, focus, active, disabled, loading, and error behavior where those
   preprocessing and no query error; all other states are red. The button's
   tooltip and accessible description retain the translated exact state. Do not
   add a separate readiness text row or change the backend readiness gate.
-- **Session groups:** one accessible multi-open accordion contains Active sessions
+- **Session groups:** one accessible single-open accordion contains Active sessions
   and Archive, both initially closed. Below the scope tabs, show only their
   adjacent section headers, without a redundant Sessions heading. Selection and
   bulk-delete controls belong inside each panel and only appear for nonempty
   lists. Select all affects that list only, preserves the other list's selection,
   and exposes a mixed state for partial selection. Preserve the existing archived
   flag as the grouping rule, independent of whether a Pi process is running.
-  Open sections share the rail's remaining height; each list scrolls independently
+  Opening a section closes the other; either can be collapsed, including both.
+  Empty lists show only the translated "No sessions yet." message.
+  The open section uses the rail's remaining height; its list scrolls internally
   with a cap of `min(18rem, 35dvh)`, while its trigger remains outside that scroll
   area. The mobile navigation dialog supplies a bounded viewport-height container.
   Keyboard users can focus and scroll each labelled panel.