feat(desktop): implement workspace foundation + desktop-client skeleton

Plan 0 (workspaces): add workspaces + workspace_memberships schema, extend
JWT/Actor/claims with primary_workspace_id, seed default workspace per tenant,
thread workspace_id through tenant monitoring quota.

Plan A (desktop skeleton): new Electron app (apps/desktop-client) with main/
preload/renderer, shared Vue component package (packages/ui-shared), and server
surface — desktop client registration + token rotation + heartbeat, SSE task
event stream, desktop accounts/tasks/content handlers, publish job endpoint,
and supporting repositories, services, sqlc queries, and migrations.

Hard cutover per plan: remove browser-extension monitoring callback endpoints,
stub legacy media API in admin-web, and delete monitoring_callback_handler.go.

apps/desktop-client/src/main/single-instance.ts

@@ -0,0 +1,21 @@
+import { app } from "electron/main";
+
+let initialized = false;
+
+export function initSingleInstance(onSecondInstance?: () => void): boolean {
+  if (initialized) {
+    return true;
+  }
+
+  initialized = true;
+  const hasLock = app.requestSingleInstanceLock();
+  if (!hasLock) {
+    return false;
+  }
+
+  app.on("second-instance", () => {
+    onSecondInstance?.();
+  });
+
+  return true;
+}