fix(migration): update systemd service file during v3→v4 migration

Critical fixes for v3→v4 migration:

1. install.sh: Auto-update systemd service file during migration
   - Calls 'nupst service enable' before restarting service
   - Only when migrating from v3 (OLD_NODE_INSTALL=1)
   - Ensures service file has correct v4 paths

2. ts/systemd.ts: Fix hardcoded v3 paths in service template
   - ExecStart: /opt/nupst/bin/nupst daemon-start (v3, broken)
              → /usr/local/bin/nupst service start-daemon (v4, correct)
   - Description: Updated to 'Deno-powered UPS Monitoring Tool'
   - Added RestartSec=10 (prevent rapid restart loops)
   - Removed NODE_ENV=production (not needed for Deno)
   - WorkingDirectory: /tmp → /opt/nupst

Without these fixes:
- Service fails to start after migration
- Service file points to non-existent /opt/nupst/bin/nupst
- Users must manually run 'nupst service enable'

Discovered via Docker migration testing in test/manualdocker/

ts/systemd.ts

@@ -15,17 +15,17 @@ export class NupstSystemd {
 
   /** Template for the systemd service file */
   private readonly serviceTemplate = `[Unit]
-Description=Node.js UPS Shutdown Tool for Multiple UPS Devices
+Description=NUPST - Deno-powered UPS Monitoring Tool
 After=network.target
 
 [Service]
-ExecStart=/opt/nupst/bin/nupst daemon-start
+ExecStart=/usr/local/bin/nupst service start-daemon
 Restart=always
+RestartSec=10
 User=root
 Group=root
 Environment=PATH=/usr/bin:/usr/local/bin
-Environment=NODE_ENV=production
-WorkingDirectory=/tmp
+WorkingDirectory=/opt/nupst
 
 [Install]
 WantedBy=multi-user.target