feat(frontend): collapsible group headers; resume/delete integration tests; English test name

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/shell/AppShell.tsx

@@ -37,6 +37,7 @@ export function AppShell() {
   const [showArchive, setShowArchive] = useState(false);
   const [renameTarget, setRenameTarget] = useState<SessionSummary | null>(null);
   const [deleteTarget, setDeleteTarget] = useState<SessionSummary | null>(null);
+  const [collapsedGroups, setCollapsedGroups] = useState<Record<string, boolean>>({});
 
   const groups = useMemo(
     () => [...new Set(sessions.map((s) => s.group).filter((g): g is string => !!g))].sort(),
@@ -195,13 +196,23 @@ export function AppShell() {
             <div className="flex flex-col gap-3">
               {groups.map((g) => (
                 <div key={g}>
-                  <p className="px-1 pb-1 text-[0.7rem] font-semibold uppercase tracking-wide text-muted-foreground">{g}</p>
-                  <NavSessions
-                    sessions={activeList.filter((s) => s.group === g)}
-                    activeSessionId={activeSessionId}
-                    onOpenPanel={openPanel}
-                    menuFor={menuFor}
-                  />
+                  <button
+                    type="button"
+                    onClick={() => setCollapsedGroups((c) => ({ ...c, [g]: !c[g] }))}
+                    aria-expanded={!collapsedGroups[g]}
+                    className="flex w-full items-center gap-1 px-1 pb-1 text-left text-[0.7rem] font-semibold uppercase tracking-wide text-muted-foreground hover:text-foreground"
+                  >
+                    <span className="select-none">{collapsedGroups[g] ? "▸" : "▾"}</span>
+                    <span className="truncate">{g}</span>
+                  </button>
+                  {!collapsedGroups[g] && (
+                    <NavSessions
+                      sessions={activeList.filter((s) => s.group === g)}
+                      activeSessionId={activeSessionId}
+                      onOpenPanel={openPanel}
+                      menuFor={menuFor}
+                    />
+                  )}
                 </div>
               ))}
               <div>