refactor(handlers): use errors.Is() for pgx error comparison in KOReader

Replace direct equality checks (err != pgx.ErrNoRows) with the idiomatic
errors.Is(err, pgx.ErrNoRows) pattern. This is the recommended Go practice
for error comparison as it correctly handles wrapped errors from error
chains, making the code more robust against future refactoring that might
wrap errors with fmt.Errorf and %w.

internal/handlers/koreader.go

@@ -4,6 +4,7 @@ import (
 	"bookhoard/internal/database"
 	wsync "bookhoard/internal/sync"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"net/http"
 	"time"
@@ -401,11 +402,11 @@ func (h *KOReaderHandler) updateProgressForBook(c *echo.Context, userID pgtype.U
 		UserID:      userID,
 	})
 
-	if err != nil && err != pgx.ErrNoRows {
+	if err != nil && !errors.Is(err, pgx.ErrNoRows) {
 		return err
 	}
 
-	hasExistingProgress := err != pgx.ErrNoRows
+	hasExistingProgress := !errors.Is(err, pgx.ErrNoRows)
 	conflictDetected := false
 
 	if hasExistingProgress && existingProgress.LastSyncSource.Valid {