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。

assets/assets.go

@@ -0,0 +1,47 @@
+// Package assets exposes the embedded PNG icon set under assets/icons.
+// Icons are exposed as raw bytes; the UI layer wraps them with Gio's
+// image decoder on demand.
+package assets
+
+import (
+	"embed"
+	"fmt"
+	"io/fs"
+
+	"git.wxccs.org/iceking2nd/winauth-go/internal/global"
+)
+
+//go:embed icons/*.png
+var iconsFS embed.FS
+
+// Icon returns the bytes of a named icon (e.g. "GoogleIcon.png"). If the
+// requested icon is missing, a Debug log is emitted and (nil, error) is
+// returned so the caller can fall back to a placeholder.
+func Icon(name string) ([]byte, error) {
+	const fn = "assets.Icon"
+	data, err := iconsFS.ReadFile("icons/" + name)
+	if err != nil {
+		global.Log.WithField("func", fn).WithField("name", name).
+			Debug("icon not found")
+		return nil, fmt.Errorf("assets: icon %q: %w", name, err)
+	}
+	return data, nil
+}
+
+// IconNames lists every embedded icon. Useful for the "choose an icon"
+// dialog in the UI.
+func IconNames() []string {
+	const fn = "assets.IconNames"
+	entries, err := fs.ReadDir(iconsFS, "icons")
+	if err != nil {
+		global.Log.WithField("func", fn).WithError(err).Warn("read embedded icons")
+		return nil
+	}
+	out := make([]string, 0, len(entries))
+	for _, e := range entries {
+		if !e.IsDir() {
+			out = append(out, e.Name())
+		}
+	}
+	return out
+}