feat: implement headless server mode with API endpoints for metrics and process management

main.go

@@ -2,6 +2,10 @@ package main
 
 import (
 	"embed"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
 
 	"github.com/wailsapp/wails/v2"
 	"github.com/wailsapp/wails/v2/pkg/options"
@@ -12,6 +16,18 @@ import (
 var assets embed.FS
 
 func main() {
+	noGUI, port, help := parseSysmonArgs()
+
+	if help {
+		printHelp()
+		os.Exit(0)
+	}
+
+	if noGUI {
+		runServer(port)
+		return
+	}
+
 	// Create an instance of the app structure
 	app := NewApp()
 
@@ -34,3 +50,56 @@ func main() {
 		println("Error:", err.Error())
 	}
 }
+
+// parseSysmonArgs scans os.Args manually so we don't call flag.Parse(),
+// which would conflict with flags that Wails injects in dev mode (e.g. -assetdir).
+func parseSysmonArgs() (noGUI bool, port int, help bool) {
+	port = 9731
+	args := os.Args[1:]
+	for i := 0; i < len(args); i++ {
+		arg := args[i]
+		switch arg {
+		case "--no-gui":
+			noGUI = true
+		case "--help", "-h":
+			help = true
+		case "--port":
+			if i+1 < len(args) {
+				i++
+				if p, err := strconv.Atoi(args[i]); err == nil && p > 0 && p <= 65535 {
+					port = p
+				}
+			}
+		default:
+			if strings.HasPrefix(arg, "--port=") {
+				if p, err := strconv.Atoi(strings.TrimPrefix(arg, "--port=")); err == nil && p > 0 && p <= 65535 {
+					port = p
+				}
+			}
+		}
+	}
+	return
+}
+
+func printHelp() {
+	fmt.Print(`sysmon — system monitor
+
+Usage:
+  sysmon                       Launch with graphical interface (default)
+  sysmon --no-gui              Run as headless HTTP API server (port 9731)
+  sysmon --no-gui --port N     Run server on port N
+
+Options:
+  --no-gui          Start without GUI, expose HTTP API instead
+  --port <number>   HTTP server port (default: 9731, requires --no-gui)
+  --help, -h        Show this help
+
+Server endpoints (--no-gui mode):
+  GET  /api/metrics              Latest metrics snapshot (JSON)
+  GET  /api/metrics/stream       Server-Sent Events stream (1 s interval)
+  GET  /api/process/<pid>        Process detail (JSON)
+  POST /api/process/<pid>/kill   Kill process  (?force=true → SIGKILL)
+  GET  /api/man/<name>           Manual page for a command
+`)
+}
+