Sleep action exposed for adopted (running-external) servers shown as 'online'

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Bug

The "Sleep" action (server card, server detail page, network context menu) is
shown/enabled whenever server.status === 'online'. But the frontend collapses
the backend's running-external lifecycle state — a server mserve only
observes/adopted, doesn't own — into that same 'online' status
(mapRuntimeStateToStatus in src/lib/server-telemetry.ts). Clicking Sleep on
an adopted server always fails.

Root cause

The backend sleep_server command
(src-tauri/src/app/commands/runtime.rs) requires LifecycleState::Online
specifically, plus an owned child process (runtime.child.is_some()). An
adopted server's backend state is RunningExternal, not Online, so the
guard rejects it with "Server must be online to sleep." — but the frontend has
no way to distinguish "owned online" from "adopted online" today, since
neither ServerRuntimeSnapshot nor the server-runtime-state event expose an
ownership flag to the UI.

Fix

Either:

• Expose an isOwned/isExternal flag on the runtime snapshot and
  server-runtime-state event so the frontend can hide Sleep (and any other
  owned-only action) for adopted servers, or
• Have sleep_server return a clearer, distinguishable error message the UI
  can surface (e.g. "This server was started outside mserve and can't be put
  to sleep"), matching the pattern already used in stop_server's
  RCON-fallback error.

Affects: src/components/server-card.tsx, src/pages/network/ServerNodeMenu.tsx,
src/pages/server/server-overview-panel.tsx.