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/i18n/locales/en/lanes.json

@@ -312,15 +312,6 @@
   "conflict.save": "Save and mark resolved",
   "changes.takeOursHint": "Keep your version of the whole file",
   "changes.takeTheirsHint": "Take the incoming version of the whole file",
-  "projects.title": "Projects",
-  "projects.folders": "Folders",
-  "projects.newWorkspace": "New workspace in {{name}}",
-  "projects.refresh": "Refresh projects and worktrees",
-  "projects.hidden_one": "{{count}} hidden",
-  "projects.hidden_other": "{{count}} hidden",
-  "projects.pick": "Pick a workspace in the sidebar",
-  "row.primary": "primary",
-  "row.adopt": "Adopt",
   "row.actions": "Workspace actions",
   "row.hint": "Double-click to rename · drag or Alt+↑/↓ to reorder · right-click for actions",
   "menu.start": "Start",
@@ -338,5 +329,10 @@
   "sessions.empty": "No sessions in this workspace yet",
   "sessions.resume": "Resume",
   "sessions.open": "Open",
-  "sessions.search": "Filter sessions…"
+  "sessions.search": "Filter sessions…",
+  "tree.title": "Workspaces",
+  "tree.search": "Search workspaces and sessions",
+  "tree.newSession": "New session",
+  "tree.noSessions": "No sessions yet",
+  "tree.transcriptHint": "No live pane — opens the transcript"
 }