feat: Phase 1 WinAuth Go 移植完整实现

将原 C#/.NET WinAuth 移植为 Go + Gio GUI，覆盖 Phase 1 全部功能。

核心模块：
- internal/authenticator: TOTP (Google/Microsoft/Okta) + HOTP + BattleNet + Steam，含
  enroll/sync/code 生成、Steam 交易确认轮询
- internal/config: YAML 配置 + 老版 WinAuth XML 导入（DPAPI + Password + Blowfish/PBKDF2 解密链）
- internal/crypto: 现代加密 (WAGO1) + DPAPI 跨平台封装 + 老版 Blowfish ECB
- internal/win32: 单实例 Mutex 锁 + 全局热键管理器 (RegisterHotKey + PeekMessage 泵) +
  SendInput Unicode 注入 + 剪贴板文本/CF_DIB 图像读写 + AttachThreadInput 焦点切换
- internal/hotkey: "Ctrl+Alt+G" 风格快捷键字符串解析/格式化
- internal/qr: gozxing 二维码解码 + otpauth:// URI 解析
- internal/i18n: en/zh-CN/de 三语 TOML

UI 模块 (Gio)：
- 主窗口：圆环倒计时进度条、复制按钮 + Toast 反馈、空列表占位、行分隔线
- 添加流程：vendor 菜单 + 各 vendor 独立对话框 + 二维码扫描入口（文件 / 剪贴板）
- 设置：密码加密、老版 XML 导入、每条目热键配置
- Steam：注册向导（含 captcha/email/SMS 多步）+ 交易确认窗

构建：Windows 主目标，非 Windows 平台所有 Win32 功能走 build-tag 桩实现。

internal/ui/toast.go

@@ -0,0 +1,84 @@
+package ui
+
+import (
+	"image"
+	"image/color"
+	"time"
+
+	"gioui.org/app"
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/op/clip"
+	"gioui.org/op/paint"
+	"gioui.org/unit"
+	"gioui.org/widget/material"
+)
+
+// toast is a transient top-of-window notification ("Copied", error
+// messages, etc). It is non-modal: drawing it does not consume pointer
+// input. Lifetime is governed by `until`; the next frame after the
+// deadline simply skips drawing.
+type toast struct {
+	msg   string
+	until time.Time
+}
+
+const toastDuration = 1500 * time.Millisecond
+
+// Show stores the message and schedules an Invalidate after the toast
+// expires so the window redraws and removes it without waiting for the
+// next user interaction.
+func (t *toast) Show(msg string, w *app.Window) {
+	t.msg = msg
+	t.until = time.Now().Add(toastDuration)
+	go func(deadline time.Time) {
+		time.Sleep(time.Until(deadline) + 50*time.Millisecond)
+		w.Invalidate()
+	}(t.until)
+}
+
+// active reports whether the toast should be drawn this frame.
+func (t *toast) active() bool {
+	return t.msg != "" && time.Now().Before(t.until)
+}
+
+// draw paints the toast as an overlay centered near the top of gtx.
+// Call AFTER laying out the rest of the frame so it stacks on top.
+func (t *toast) draw(gtx layout.Context, th *material.Theme) {
+	if !t.active() {
+		return
+	}
+	bg := color.NRGBA{R: 0x20, G: 0x20, B: 0x20, A: 0xe0}
+	fg := color.NRGBA{R: 0xff, G: 0xff, B: 0xff, A: 0xff}
+
+	macro := op.Record(gtx.Ops)
+	pad := layout.UniformInset(unit.Dp(10))
+	dims := pad.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
+		lbl := material.Body2(th, t.msg)
+		lbl.Color = fg
+		return lbl.Layout(gtx)
+	})
+	call := macro.Stop()
+
+	// Center horizontally near the top.
+	x := (gtx.Constraints.Max.X - dims.Size.X) / 2
+	if x < 0 {
+		x = 0
+	}
+	y := gtx.Dp(unit.Dp(12))
+
+	stack := op.Offset(image.Point{X: x, Y: y}).Push(gtx.Ops)
+	rrect := clip.RRect{
+		Rect: image.Rectangle{Max: dims.Size},
+		SE:   gtx.Dp(unit.Dp(6)),
+		SW:   gtx.Dp(unit.Dp(6)),
+		NE:   gtx.Dp(unit.Dp(6)),
+		NW:   gtx.Dp(unit.Dp(6)),
+	}
+	bgArea := rrect.Push(gtx.Ops)
+	paint.ColorOp{Color: bg}.Add(gtx.Ops)
+	paint.PaintOp{}.Add(gtx.Ops)
+	bgArea.Pop()
+	call.Add(gtx.Ops)
+	stack.Pop()
+}