feat(updates): auto-apply and self-restart on new dashboard versions

Adds POST /api/updates/apply (pull, rebuild, restart) plus an "Update now"
button and hourly auto-check in the UI, for checkouts that are safely
fast-forwardable. Reintroduces self-restart (previously removed in edc25ca
for cross-environment reliability concerns) per explicit user request.

client/src/i18n/locales/en/updates.json

@@ -11,5 +11,10 @@
   "restartNote": "After the command finishes, close and restart the dashboard the same way you started it: if you used npm start, run npm start again.",
   "checkNow": "Check now",
   "checking": "Checking...",
-  "checkError": "Could not check for updates"
+  "checkError": "Could not check for updates",
+  "updateNow": "Update now",
+  "updating": "Updating...",
+  "restarting": "Rebuilt — restarting the dashboard...",
+  "applyError": "Could not apply the update",
+  "reasonNotFastForwardable": "This checkout isn't on the tracked default branch — apply the command above manually."
 }