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'.

server/src/app.ts

@@ -73,7 +73,7 @@ export async function buildApp() {
 
   fastify.get('/api/health', async () => ({
     status: 'ok',
-    version: '0.4.7',
+    version: '0.4.8',
     timestamp: new Date().toISOString(),
   }))