fix(login): fetch version from /api/health instead of hardcoding, bump v0.4.8

Login page had 'NixServer v0.3.2 · Port 2087' hardcoded and subtitle
said 'Web Host Manager'. Now fetches version from GET /api/health on
mount (no auth needed) and shows it dynamically. Changed subtitle to
'Admin Panel'.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "nixpanel-server",
-  "version": "0.4.7",
+  "version": "0.4.8",
   "private": true,
   "workspaces": [
     "server",