feat(ui): 主题切换 + 语言持久化 + About 对话框

- 新增 light/dark 主题调色板与 Windows 注册表系统主题探测
- 偏好设置对话框：主题三选一 + 语言下拉(en/zh-CN/de)，即时生效
- 持久化 Language/Theme/AutoLockMinutes/MinimizeToTray 至 YAML 顶层
- 启动时按 config 中保存的语言初始化 i18n
- About 对话框展示 version/runtime/项目地址/许可证
- 全部硬编码颜色迁移到 themePalette，为深色模式做准备

internal/ui/theme_windows.go

@@ -0,0 +1,25 @@
+//go:build windows
+
+package ui
+
+import "golang.org/x/sys/windows/registry"
+
+// systemPrefersDark queries the Windows personalization registry key
+// to find out whether the user picked dark mode for apps. Returns
+// false on any read error so the default stays "light".
+func systemPrefersDark() bool {
+	k, err := registry.OpenKey(
+		registry.CURRENT_USER,
+		`Software\Microsoft\Windows\CurrentVersion\Themes\Personalize`,
+		registry.QUERY_VALUE,
+	)
+	if err != nil {
+		return false
+	}
+	defer k.Close()
+	v, _, err := k.GetIntegerValue("AppsUseLightTheme")
+	if err != nil {
+		return false
+	}
+	return v == 0
+}