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/i18n/en.js

@@ -529,9 +529,13 @@ export default {
   "keys.snip.withKey": "Snippets below include the key you just created.",
   "keys.snip.openMode": "No API keys are configured — the gateway accepts any key value.",
   "keys.snip.keyPh": "your API key",
-  "settings.restart": "Restart gateway",
-  "settings.restartConfirm": "Restart the gateway? Requests in flight will be dropped; local model servers restart too.",
+  "settings.restart": "Restart everything",
+  "settings.restartConfirm": "Restart the gateway and all local model servers? Requests in flight will be dropped.",
   "settings.restarting": "Restarting — the page reloads when the gateway is back…",
+  "settings.shutdown": "Stop everything & power off",
+  "settings.shutdownConfirm": "Stop all local model servers and power the gateway off? The API and this panel stop answering until you start the gateway again.",
+  "settings.shutdownDone": "The gateway is off.",
+  "settings.shutdownHint": "All local model servers are stopped and the process has exited. Start the gateway again on the server to bring the panel back.",
   "bench.title": "Local model speed",
   "bench.hint": "Runs `cortiq bench` on this machine — real tok/s, one model at a time.",
   "bench.run": "Measure",