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/vendor_menu.go

@@ -1,8 +1,6 @@
 package ui
 
 import (
-	"image/color"
-
 	"gioui.org/layout"
 	"gioui.org/unit"
 	"gioui.org/widget"
@@ -71,7 +69,7 @@ func (m *vendorMenu) Pick(gtx layout.Context) (vendor, bool) {
 }
 
 func (m *vendorMenu) Layout(gtx layout.Context, th *material.Theme) layout.Dimensions {
-	fillBackground(gtx, color.NRGBA{R: 0, G: 0, B: 0, A: 0x60})
+	fillBackground(gtx, activePalette.ScrimBg)
 
 	row := func(btn *widget.Clickable, label string) layout.FlexChild {
 		return layout.Rigid(func(gtx layout.Context) layout.Dimensions {
@@ -86,11 +84,11 @@ func (m *vendorMenu) Layout(gtx layout.Context, th *material.Theme) layout.Dimen
 	return layout.Center.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 		gtx.Constraints.Max.X = gtx.Dp(320)
 		return widget.Border{
-			Color:        color.NRGBA{R: 0x55, G: 0x55, B: 0x55, A: 0xff},
+			Color:        activePalette.DialogBorder,
 			CornerRadius: unit.Dp(4),
 			Width:        unit.Dp(1),
 		}.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
-			fillBackground(gtx, color.NRGBA{R: 0xff, G: 0xff, B: 0xff, A: 0xff})
+			fillBackground(gtx, activePalette.DialogBg)
 			return layout.UniformInset(unit.Dp(16)).Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 				return layout.Flex{Axis: layout.Vertical}.Layout(gtx,
 					layout.Rigid(material.H6(th, i18n.T("menu_choose_vendor")).Layout),