Add local PromptKit backend configuration

internal/core/config/validation.go

@@ -2,6 +2,7 @@ package config
 
 import (
 	"fmt"
+	"net/url"
 	"sort"
 	"strings"
 
@@ -53,6 +54,27 @@ func validatePromptKit(cfg PromptKitConfig) error {
 	if strings.TrimSpace(cfg.ProfileDir) != "" && strings.TrimSpace(cfg.ProfileFile) != "" {
 		return fmt.Errorf("promptkit profile_dir and profile_file are mutually exclusive")
 	}
+	if cfg.LocalBackend == nil {
+		return nil
+	}
+	endpoint := strings.TrimSpace(cfg.LocalBackend.Endpoint)
+	if endpoint == "" {
+		return fmt.Errorf("promptkit.local_backend.endpoint must not be empty when set")
+	}
+	parsed, err := url.Parse(endpoint)
+	if err != nil ||
+		(!strings.EqualFold(parsed.Scheme, "http") && !strings.EqualFold(parsed.Scheme, "https")) ||
+		!parsed.IsAbs() ||
+		parsed.Hostname() == "" ||
+		parsed.User != nil ||
+		parsed.RawQuery != "" ||
+		parsed.ForceQuery ||
+		strings.Contains(endpoint, "#") {
+		return fmt.Errorf("promptkit.local_backend.endpoint must be an absolute HTTP or HTTPS URL with a host and no user information, query, or fragment")
+	}
+	if cfg.LocalBackend.ConcurrencyLimit < 0 {
+		return fmt.Errorf("promptkit.local_backend.concurrency_limit must not be negative")
+	}
 	return nil
 }