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.

server/internal/shared/auth/context.go

@@ -3,9 +3,10 @@ package auth
 import "context"
 
 type Actor struct {
-	UserID   int64
-	TenantID int64
-	Role     string
+	UserID             int64
+	TenantID           int64
+	PrimaryWorkspaceID int64
+	Role               string
 }
 
 type actorKey struct{}