fix: restart backend under systemd and use .sproutclaw agent dir

Restart via systemctl when running as a service, set SPROUTCLAW_CODING_AGENT_DIR
for the RPC subprocess, and add a frontend poll timeout for failed restarts.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/internal/handlers/settings.go

@@ -2,10 +2,8 @@ package handlers
 
 import (
 	"fmt"
-	"log"
 	"net/http"
 	"os"
-	"os/exec"
 	"runtime"
 	"strings"
 	"sync"
@@ -37,28 +35,6 @@ func RegisterSettings(r *gin.RouterGroup, cfg *config.Config, database *db.DB, p
 
 var processStartTime = time.Now()
 
-func handleRestart() gin.HandlerFunc {
-	return func(c *gin.Context) {
-		c.JSON(http.StatusOK, gin.H{"ok": true})
-		go func() {
-			time.Sleep(300 * time.Millisecond)
-			exe, err := os.Executable()
-			if err != nil {
-				log.Printf("[restart] get executable: %v", err)
-				os.Exit(1)
-			}
-			cmd := exec.Command(exe, os.Args[1:]...)
-			cmd.Stdout = os.Stdout
-			cmd.Stderr = os.Stderr
-			if err := cmd.Start(); err != nil {
-				log.Printf("[restart] start new process: %v", err)
-				os.Exit(1)
-			}
-			os.Exit(0)
-		}()
-	}
-}
-
 func handleGetSettings(cfg *config.Config, database *db.DB) gin.HandlerFunc {
 	return func(c *gin.Context) {
 		systemPrompt, _ := services.ReadSystemPrompt(cfg.SystemPromptFile)