rebuild the chat session list on the shared data primitives

the list drew its own border per row on top of nothing, so every boundary
between two rows was a double hairline, and it emphasised two things per row
where the design language allows one. DataRow/DataList settle both.

loading and empty were the same grey sentence, which made a slow transcript
read look like an account with no history; they are now LoadingBlock and
EmptyBlock, and a failed read gets an ErrorBlock with the actual message
instead of rendering as "no sessions".

rename and delete swallowed their failures whole — useClient only raises a
dialog for 401, 403 and 5xx, and the likely error here is a 404 from a
transcript that vanished under you. both toast now, as does a deep link to a
session that cannot be read, which used to open an empty pane and say nothing.

the New Chat button was duck-teal filled with duck-yellow text: duck-teal is
a bright cyan in dark mode and duck-yellow has no dark override, so the pair
sat near 2:1 contrast in both themes.

active-row highlight now comes from the route rather than the selection
channel, so it is right on a deep link before any panel has published, and
deleting the open session navigates out of it instead of leaving a dead route.

deletes SessionBar and SessionContextMenu: the first was exported through two
barrels and imported nowhere, the second was never imported at all and typed
its session id as a number.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/workspaces/state/src/useClaudeSessions.ts

@@ -62,7 +62,7 @@ export function useClaudeSessions(cwd?: string | null) {
   const { isAuthenticated } = useAuth();
   const q = cwdQuery(cwd);
 
-  const { data, isLoading, refetch } = useQuery<{ sessions: ClaudeSessionSummary[] }>({
+  const { data, isLoading, error, refetch } = useQuery<{ sessions: ClaudeSessionSummary[] }>({
     queryKey: [SESSIONS_KEY, cwd ?? 'default'],
     enabled: isAuthenticated,
     queryFn: () => client.get<{ sessions: ClaudeSessionSummary[] }>(`/chat/sessions${q}`),
@@ -93,5 +93,16 @@ export function useClaudeSessions(cwd?: string | null) {
     [client, q, invalidate],
   );
 
-  return { sessions: data?.sessions ?? [], isLoading, refetch, loadSession, deleteSession, renameSession, invalidate };
+  return {
+    sessions: data?.sessions ?? [],
+    isLoading,
+    // Surfaced so the list can distinguish "no sessions" from "the read failed" — they rendered
+    // identically before, and the second one is the only one you can act on.
+    error,
+    refetch,
+    loadSession,
+    deleteSession,
+    renameSession,
+    invalidate,
+  };
 }