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/keep-alive.ts

@@ -0,0 +1,18 @@
+interface KeepAlivePlan {
+  mode: "probe-only" | "heartbeat" | "disabled";
+  updatedAt: number;
+}
+
+const keepAlivePlan: KeepAlivePlan = {
+  mode: "probe-only",
+  updatedAt: Date.now(),
+};
+
+export function updateKeepAlivePlan(mode: KeepAlivePlan["mode"]): void {
+  keepAlivePlan.mode = mode;
+  keepAlivePlan.updatedAt = Date.now();
+}
+
+export function getKeepAlivePlan(): KeepAlivePlan {
+  return { ...keepAlivePlan };
+}