Fix go vet IPv6 address format errors in email notifier and server

Replace fmt.Sprintf("%s:%d") with net.JoinHostPort() for IPv6 compatibility.
Bump setup-go action to v5 to resolve Node.js 20 deprecation warnings.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cmd/server/main.go

@@ -4,9 +4,11 @@ import (
 	"context"
 	"fmt"
 	"log/slog"
+	"net"
 	"net/http"
 	"os"
 	"os/signal"
+	"strconv"
 	"syscall"
 	"time"
 
@@ -241,7 +243,7 @@ func main() {
 	}
 
 	// Server configuration
-	addr := fmt.Sprintf("%s:%d", cfg.Server.Host, cfg.Server.Port)
+	addr := net.JoinHostPort(cfg.Server.Host, strconv.Itoa(cfg.Server.Port))
 	httpServer := &http.Server{
 		Addr:         addr,
 		Handler:      finalHandler,