fix: sanitize sync error messages, detect non-sync servers, add health check in TestAuth

internal/core/sync/client.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
+	"strings"
 	"time"
 )
 
@@ -124,12 +125,35 @@ func (c *Client) RevokeCurrent() error {
 
 // TestAuth checks credentials without creating a device.
 func (c *Client) TestAuth(serverURL, username, password string) error {
-	body := map[string]string{"username": username, "password": password}
+	// First, check if this is a Verstak Sync server
+	healthURL := strings.TrimSuffix(serverURL, "/") + "/api/v1/health"
+	req, err := http.NewRequest("GET", healthURL, nil)
+	if err != nil {
+		return fmt.Errorf("invalid URL: %w", err)
+	}
+	resp, err := c.HTTP.Do(req)
+	if err != nil {
+		return fmt.Errorf("connection failed: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		return fmt.Errorf("not a Verstak Sync server (HTTP %d)", resp.StatusCode)
+	}
+
+	data, _ := io.ReadAll(resp.Body)
+	body := string(data)
+	if !strings.Contains(body, "status") && !strings.Contains(body, "ok") {
+		return fmt.Errorf("not a Verstak Sync server (unexpected response)")
+	}
+
+	// Now test actual auth
+	authBody := map[string]string{"username": username, "password": password}
 	savedURL := c.ServerURL
 	savedKey := c.APIKey
 	c.ServerURL = serverURL
 	c.APIKey = ""
-	err := c.post("/api/auth/test", body, nil)
+	err = c.post("/api/auth/test", authBody, nil)
 	c.ServerURL = savedURL
 	c.APIKey = savedKey
 	return err
@@ -273,6 +297,13 @@ func (c *Client) DownloadBlob(sha256, destPath string) error {
 	return err
 }
 
+func minInt(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
 func (c *Client) bearerToken() string {
 	if c.DeviceToken != "" {
 		return c.DeviceToken
@@ -301,8 +332,7 @@ func (c *Client) post(path string, body, result interface{}) error {
 	defer resp.Body.Close()
 
 	if resp.StatusCode >= 400 {
-		data, _ := io.ReadAll(resp.Body)
+		return c.readErrorBody(resp, resp.StatusCode)
-		return fmt.Errorf("server %d: %s", resp.StatusCode, string(data))
 	}
 
 	if result != nil {
@@ -325,8 +355,7 @@ func (c *Client) get(path string, result interface{}) error {
 	defer resp.Body.Close()
 
 	if resp.StatusCode >= 400 {
-		data, _ := io.ReadAll(resp.Body)
+		return c.readErrorBody(resp, resp.StatusCode)
-		return fmt.Errorf("server %d: %s", resp.StatusCode, string(data))
 	}
 
 	if result != nil {
@@ -334,3 +363,15 @@ func (c *Client) get(path string, result interface{}) error {
 	}
 	return nil
 }
+
+func (c *Client) readErrorBody(resp *http.Response, statusCode int) error {
+	buf := make([]byte, 4096)
+	n, _ := io.ReadFull(resp.Body, buf)
+	body := string(buf[:minInt(n, 500)])
+
+	lower := strings.ToLower(body)
+	if strings.Contains(lower, "<html") || strings.Contains(lower, "<!doctype") {
+		return fmt.Errorf("not a Verstak Sync server (HTTP %d)", statusCode)
+	}
+	return fmt.Errorf("server error (HTTP %d)", statusCode)
+}