fix: auto-retry version fetch + add uptime/kill to exec allowlist

Layout.tsx: when versionError is true, automatically retry fetchVersion
after 5 s. This handles the case where the initial fetch runs during
the service-restart window after an upgrade (server not yet up), which
caused the sidebar footer to permanently show 'Version unavailable'
even though the server came back online shortly after.

exec.ts: add 'uptime' and 'kill' to ALLOWED_COMMANDS. 'uptime' was
missing, causing exec('uptime', ['-p']) in GET /api/nixserver/system/info
to throw 'Command not allowed', which rejected the entire Promise.all
and returned 500 — leaving Hostname and OS as '...' on the dashboard.

server/src/core/exec.ts

@@ -24,7 +24,7 @@ const ALLOWED_COMMANDS = new Set([
   // Network
   'ss', 'ip', 'hostname',
   // System
-  'uname', 'free', 'ps',
+  'uname', 'uptime', 'free', 'ps', 'kill',
   // PHP
   'php8.2', 'php8.3',
   // Fail2ban