feat: add host restart detection and fix agent version display

Potential fixes for issues #4 and #6.

Agent version display:
- Set CurrentVersion during registration instead of waiting for first check-in
- Update UI to show "Initial Registration" instead of "Unknown"

Host restart detection:
- Added reboot_required, last_reboot_at, reboot_reason fields to agents table
- Agent now detects pending reboots (Debian/Ubuntu via /var/run/reboot-required, RHEL/Fedora via needs-restarting)
- New reboot command type with 1-minute grace period
- UI shows restart alerts and adds restart button in quick actions
- Restart indicator badge in agent list

The reboot detection runs during system info collection and gets reported back to the server automatically.

Using shutdown command for now until we make the restart mechanism user-adjustable later - need to think on that.
Also need to come up with a Windows derivative outside of reading event log for detecting reboots.

aggregator-server/cmd/server/main.go

@@ -206,6 +206,7 @@ func main() {
 			dashboard.POST("/agents/:id/update", agentHandler.TriggerUpdate)
 			dashboard.POST("/agents/:id/heartbeat", agentHandler.TriggerHeartbeat)
 			dashboard.GET("/agents/:id/heartbeat", agentHandler.GetHeartbeatStatus)
+			dashboard.POST("/agents/:id/reboot", agentHandler.TriggerReboot)
 			dashboard.GET("/updates", updateHandler.ListUpdates)
 			dashboard.GET("/updates/:id", updateHandler.GetUpdate)
 			dashboard.GET("/updates/:id/logs", updateHandler.GetUpdateLogs)