fix(theme): 修复主题管理系统逻辑缺陷

- removeTheme() 删除当前活动主题后从 ConfigManager 读取已保存的主题偏好作为回退方案，不再硬编码 'system'
- saveAndApply() 在调用 _applyCustomTheme 前先 setActiveStyle(style)，确保主题应用时使用最新选择的内置样式
- _applyCustomTheme() 返回 bool 表示成败，失败时调用方清除配置中的 custom_theme 避免下次启动循环失败
- importTheme() 增加 self.__lock 保护，消除 TOCTOU 竞态条件
- ThemeManager 新增 CfgKey 导入以支持 removeTheme 读取配置

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/gui/ALSettingsWidget.py

@@ -34,6 +34,7 @@ import managers.config.ConfigManager as ConfigManager
 from managers.log.LogManager import instance as logInstance
 from managers.theme.ThemeManager import(
     getActiveStyle,
+    setActiveStyle,
     instance as themeInstance
 )
 
@@ -47,18 +48,20 @@ from interfaces.ConfigProvider import (
 def _applyCustomTheme(
     name: str,
     fallback_theme: str = "system"
-):
+) -> bool:
 
     if not name:
         themeInstance().clearTheme(fallback_theme)
-        return
+        return True
     try:
         themeInstance().applyTheme(name)
+        return True
     except Exception as e:
         logInstance().getLogger("ALSettingsWidget").warning(
             f"无法应用自定义主题 '{name}'，回退到 {fallback_theme} 外观: {e}"
         )
         themeInstance().clearTheme(fallback_theme)
+        return False
 
 def _themeToReadable(
     need_theme: str
@@ -266,7 +269,9 @@ class ALSettingsWidget(QWidget, Ui_ALSettingsWidget):
         theme, style, custom_theme = self.collectSettings()
         self.__cfg_mgr.set(CfgKey.GLOBAL.APPEARANCE.STYLE, style)
         self.__cfg_mgr.set(CfgKey.GLOBAL.APPEARANCE.CUSTOM_THEME, custom_theme)
-        _applyCustomTheme(custom_theme, theme)
+        setActiveStyle(style)
+        if not _applyCustomTheme(custom_theme, theme):
+            self.__cfg_mgr.set(CfgKey.GLOBAL.APPEARANCE.CUSTOM_THEME, "")
         self.syncRadioFromNeedTheme(custom_theme)
         # Re-read theme after syncRadioFromNeedTheme — the radio may have
         # changed to match the custom theme's need_theme