fix(settings): apply a theme as soon as it is selected and restore it on cancel

PLAN 5.6 asks for the theme to take effect immediately, but it was applied
only on OK. Selecting a theme now previews it; OK saves it, and cancel, Escape
or closing the window restores the theme the dialog opened with.

docs/ACCEPTANCE.md

@@ -30,7 +30,8 @@
 
 ### A3 主题三态切换
 
-- [ ] 设置 → 主题 亮色 / 暗色 / 跟随系统 依次切换：**即时生效**，无重启提示
+- [ ] 设置 → 主题 亮色 / 暗色 / 跟随系统 依次切换：下拉框一选中就**即时生效**（无需点确认），无重启提示
+- [ ] 切换主题后点取消、按 Esc 或点标题栏关闭：恢复为打开设置时的主题；重启后仍是原主题
 - [ ] 每种主题下检查：主窗口、看板列与卡片、标签 tab、全部模态、全部错误页颜色完整切换，无亮底暗字/暗底亮字残留
 - [ ] 标签徽章在两种主题下均显示原色（§5.9 例外），文字颜色保持可读
 - [ ] 跟随系统：程序运行中切换 Windows 深浅色 → 程序跟随变化