feat(admin): power controls — stop everything & power off, restart everything

Settings gains a second service button next to restart: stop every managed
local model server and exit the gateway process (nothing is respawned; in
Docker the restart policy decides). The SPA replaces itself with a static
"gateway is off" screen since the API is gone. The restart button is
relabeled "Restart everything" to say what it always did — the gateway
re-execs itself and managed servers come back with it. Translated in all
seven languages.

web/js/api.js

@@ -134,6 +134,7 @@ export const api = {
   logs: (limit = 300) => j("GET", "/logs?limit=" + limit),
   clearLogs: () => j("DELETE", "/logs"),
   restart: () => j("POST", "/restart"),
+  shutdown: () => j("POST", "/shutdown"),
   benchStart: (model) => j("POST", "/bench", { model }),
   benchList: () => j("GET", "/bench"),