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/lib/projectTreePrefs.ts

@@ -1,7 +1,8 @@
 /**
- * @file projectTreePrefs — the sidebar project tree's viewer preferences, in
- * ONE localStorage key (`agenthub.sidebar.projects`): which projects are
- * collapsed, whether a repo's untracked worktrees are listed, and which
+ * @file projectTreePrefs — the sidebar workspace list's viewer preferences, in
+ * ONE localStorage key (`agenthub.sidebar.projects`): which workspaces are
+ * expanded to their sessions (by lane id), whether a repo's untracked
+ * worktrees are offered, and which
  * worktree sources (`claude` = `<repo>/.claude/worktrees/*`, `agenthub` =
  * LANES_ROOT) are hidden — globally, or per project root, where a project's own
  * list (Settings → Projects) replaces the global one. A `useSyncExternalStore` reader that caches nothing
@@ -15,7 +16,8 @@ import { useSyncExternalStore } from "react";
 export const PROJECT_TREE_KEY = "agenthub.sidebar.projects";
 
 export interface ProjectTreePrefs {
-  collapsed: Record<string, boolean>;
+  /** Lane id → its sessions are shown under it in the sidebar. */
+  expanded: Record<string, boolean>;
   showUntracked: boolean;
   hideSources: string[];
   /** Per project root: replaces `hideSources` for that project. */
@@ -23,7 +25,7 @@ export interface ProjectTreePrefs {
 }
 
 const DEFAULTS: ProjectTreePrefs = {
-  collapsed: {},
+  expanded: {},
   showUntracked: true,
   hideSources: [],
   projectHideSources: {},
@@ -46,7 +48,7 @@ export function readProjectTreePrefs(): ProjectTreePrefs {
   try {
     const parsed = raw ? (JSON.parse(raw) as Partial<ProjectTreePrefs>) : {};
     value = {
-      collapsed: parsed.collapsed && typeof parsed.collapsed === "object" ? parsed.collapsed : {},
+      expanded: parsed.expanded && typeof parsed.expanded === "object" ? parsed.expanded : {},
       showUntracked: parsed.showUntracked !== false,
       hideSources: Array.isArray(parsed.hideSources) ? parsed.hideSources : [],
       projectHideSources: