refactor(release): bump to 1.6.0 and unify version source

main.py

@@ -11,6 +11,7 @@ import re
 import threading
 import urllib.error
 import urllib.request
+from app_version import APP_VERSION
 from PySide6.QtWidgets import (QApplication, QMainWindow, QLabel, QLineEdit,
                                QPushButton, QVBoxLayout, QWidget, QMessageBox,
                                QTextBrowser, QScrollArea, QCheckBox, QHBoxLayout,
@@ -33,7 +34,6 @@ LOG_FILE = os.path.join(APP_DATA_DIR, "app.log")
 LOG_MAX_BYTES = 1024 * 1024  # 1 MB
 LOG_BACKUP_FILE = os.path.join(APP_DATA_DIR, "app.log.1")
 AUTH_RELOGIN_BACKOFF_SECONDS = 5.0
-APP_VERSION = "1.5.1"
 # Legacy owner/repo format for GitHub-only fallback.
 UPDATE_REPOSITORY = ""
 # Full repository URL is preferred (supports GitHub/Gitea).