Gate workspace_update emissions to prevent unnecessary sidebar re-renders

Server: remove activityAt computation from workspace descriptors (always
sends null for backwards compat) and deduplicate emissions in
emitWorkspaceUpdatesForWorkspaceIds using fast-deep-equal — workspace
updates are now only sent when the descriptor actually changed.

Client: remove activityAt from WorkspaceDescriptor, sidebar entry types,
and baseline sort comparators. Workspace/project ordering is fully
handled by the persisted sidebar order store.

packages/server/package.json

@@ -76,6 +76,7 @@
     "dotenv": "^17.2.3",
     "express": "^4.18.2",
     "express-basic-auth": "^1.2.1",
+    "fast-deep-equal": "^3.1.3",
     "fast-uri": "^3.1.0",
     "mnemonic-id": "^3.2.7",
     "node-pty": "1.2.0-beta.11",