feat(ui): sidebar lists workspaces with their sessions, Claude Desktop style

The sidebar's project tree becomes a flat workspace list (WorkspaceTree):
one row per workspace in the stored order, each expanding to its sessions.
A session that still has a live pane opens /workspace?lane=<id>&run=<runId>,
which only attaches pane 0 to that tmux session; any other session opens
its read-only transcript. Nothing in the sidebar starts or resumes a run.

client/src/components/Sidebar.tsx

@@ -78,7 +78,7 @@ import type { LucideIcon } from "lucide-react";
 import { api } from "../lib/api";
 import { eventBus } from "../lib/eventBus";
 import { useTheme } from "../hooks/useTheme";
-import { ProjectTree } from "./ProjectTree";
+import { WorkspaceTree } from "./WorkspaceTree";
 import type { UpdateStatusPayload, WSMessage } from "../lib/types";
 
 function isUpdatePayload(x: unknown): x is UpdateStatusPayload {
@@ -472,20 +472,20 @@ export function Sidebar({
               </NavLink>
             );
           })}
-          {/* The project → workspace tree scrolls with the nav. In the rail it
+          {/* The workspace → sessions list scrolls with the nav. In the rail it
               is one icon that opens the sidebar instead. */}
           {collapsed ? (
             <button
               type="button"
               onClick={onToggle}
-              title={t("lanes:projects.title")}
-              aria-label={t("lanes:projects.title")}
+              title={t("lanes:tree.title")}
+              aria-label={t("lanes:tree.title")}
               className="flex w-full items-center justify-center rounded-lg border border-transparent px-2 py-2.5 text-fg-secondary transition-colors hover:bg-surface-3 hover:text-fg-primary"
             >
               <FolderTree className="w-4 h-4" />
             </button>
           ) : (
-            <ProjectTree />
+            <WorkspaceTree />
           )}
         </nav>
         {!collapsed && navOverflow.up && (