feat(nixserver): version display and one-click panel upgrade

Add version badge to sidebar footer showing installed vs latest version,
fetched from /api/nixserver/system/version. Badge shows green when up to
date or amber with target version when an update is available.

Upgrade button triggers POST /api/nixserver/system/upgrade which spawns
scripts/update.sh detached (git pull + npm install + prisma migrate +
build + restart). Response is returned before the service restarts so
the client receives confirmation. Panel comes back online in ~60 s.

server/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@nixpanel/server",
-  "version": "0.4.2",
+  "version": "0.4.3",
   "private": true,
   "type": "module",
   "main": "dist/index.js",