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/pages/Workspace.tsx

@@ -44,7 +44,7 @@
  * ----------------------------------------------------------------------------- */
 
 import { useCallback, useEffect, useMemo, useState, useSyncExternalStore } from "react";
-import { useSearchParams } from "react-router-dom";
+import { useLocation, useSearchParams } from "react-router-dom";
 import { useTranslation } from "react-i18next";
 import {
   Plus,
@@ -231,6 +231,7 @@ function ConsoleArea({
   onLaneIdChange,
   showToggle = true,
   resumeRequest,
+  attachRequest,
 }: {
   lanes: Lane[];
   selectedLaneId: number | null;
@@ -248,6 +249,8 @@ function ConsoleArea({
   showToggle?: boolean;
   /** A session to resume in slot 0, from the Sessions tab. */
   resumeRequest: { sessionId: string; model: string | null; nonce: number } | null;
+  /** A live run to attach slot 0 to, from the sidebar (`?run=`). */
+  attachRequest: { runId: string; nonce: string } | null;
 }) {
   // Every split pane shows the SELECTED lane, each in its own tmux session
   // (slot = pane index). Tracking what each pane shows lets the others leave
@@ -292,6 +295,7 @@ function ConsoleArea({
           defaultCwd={defaultCwd}
           onHasActiveRunChange={onHasActiveRunChange}
           resumeRequest={resumeRequest}
+          attachRequest={attachRequest}
         />
       ) : (
         <div
@@ -323,6 +327,7 @@ function ConsoleArea({
                 wsConnected={wsConnected}
                 defaultCwd={defaultCwd}
                 resumeRequest={i === 0 ? resumeRequest : null}
+                attachRequest={i === 0 ? attachRequest : null}
               />
             </div>
           ))}
@@ -342,6 +347,14 @@ export function Workspace() {
   // notification click), falling back to the last one shown, then the first.
   const { lanes, loaded: lanesLoaded, refresh: refreshLanes } = useLanes();
   const laneParam = searchParams.get("lane");
+  // `?run=` — a live session picked in the sidebar; the location key makes a
+  // second pick of the same session a new request.
+  const location = useLocation();
+  const runParam = searchParams.get("run");
+  const attachRequest = useMemo(
+    () => (runParam ? { runId: runParam, nonce: location.key } : null),
+    [runParam, location.key]
+  );
   const paramId = laneParam === null ? null : Number(laneParam);
   const fallbackId = (() => {
     if (!lanesLoaded) return null;
@@ -948,6 +961,7 @@ export function Workspace() {
                 onLaneIdChange={setSelectedLaneId}
                 showToggle={!currentLane}
                 resumeRequest={resumeRequest}
+                attachRequest={attachRequest}
               />
             </div>
           </section>