feat(ui): 安全增强 — 修改密码 / 自动锁定 / HOTP 提示 / 配置备份

- 修改密码对话框引入旧密码校验：通过 store.VerifyPassword 走 constant-time
  比较，对话框自身不持有 live passphrase 副本。
- 配置加密且 auto_lock_minutes>0 时启用自动锁定；明文配置不受影响。
  锁定时刻意保留 store.passphrase，避免解锁瞬间出现保存竞态——进程内存
  无论如何都是威胁边界。
- HOTP 点击后 toast 仅显示推进后的计数器值，绝不携带验证码字符串。
- SaveYAML 每次写入前将旧文件轮转为 .bak(0o600)，防止加密/序列化失败
  导致用户无可恢复副本。
- Preferences 对话框新增"自动锁定（分钟）"输入项。

internal/ui/dialog_change_password.go

@@ -0,0 +1,88 @@
+package ui
+
+import (
+	"crypto/subtle"
+
+	"gioui.org/layout"
+	"gioui.org/unit"
+	"gioui.org/widget"
+	"gioui.org/widget/material"
+
+	"git.wxccs.org/iceking2nd/winauth-go/internal/i18n"
+)
+
+// changePasswordDialog asks for the current passphrase, then a new
+// passphrase + confirmation. The dialog refuses to submit unless the
+// current passphrase matches what the store already holds (via a
+// constant-time compare to avoid leaking length / prefix information).
+type changePasswordDialog struct {
+	oldEd     widget.Editor
+	newEd     widget.Editor
+	confirmEd widget.Editor
+	okBtn     widget.Clickable
+	cancelBtn widget.Clickable
+	errorMsg  string
+}
+
+func newChangePasswordDialog() *changePasswordDialog {
+	d := &changePasswordDialog{}
+	d.oldEd.SingleLine = true
+	d.oldEd.Mask = '*'
+	d.newEd.SingleLine = true
+	d.newEd.Mask = '*'
+	d.confirmEd.SingleLine = true
+	d.confirmEd.Mask = '*'
+	return d
+}
+
+// changePasswordResult tells the caller what to do. cancel skips any
+// state change. ok=true means commit newPw via store.SetPassword.
+type changePasswordResult struct {
+	cancel bool
+	newPw  []byte
+}
+
+// Layout takes a verify callback that returns true when oldPw matches
+// what the store believes the current passphrase is. The caller is
+// responsible for that comparison so the dialog itself never has to
+// hold a copy of the live passphrase.
+func (d *changePasswordDialog) Layout(
+	gtx layout.Context, th *material.Theme,
+	verifyOld func(oldPw []byte) bool,
+	onDone func(changePasswordResult),
+) layout.Dimensions {
+	if d.cancelBtn.Clicked(gtx) {
+		onDone(changePasswordResult{cancel: true})
+		return layout.Dimensions{Size: gtx.Constraints.Max}
+	}
+	if d.okBtn.Clicked(gtx) {
+		oldPw := []byte(d.oldEd.Text())
+		newPw := []byte(d.newEd.Text())
+		confirm := []byte(d.confirmEd.Text())
+		switch {
+		case !verifyOld(oldPw):
+			// Use the constant-length wrong-password string so the
+			// presence of an error is not itself a side channel.
+			d.errorMsg = i18n.T("msg_password_wrong")
+		case subtle.ConstantTimeCompare(newPw, confirm) != 1:
+			d.errorMsg = i18n.T("msg_password_mismatch")
+		default:
+			onDone(changePasswordResult{newPw: newPw})
+			return layout.Dimensions{Size: gtx.Constraints.Max}
+		}
+	}
+
+	body := func(gtx layout.Context) layout.Dimensions {
+		return layout.Flex{Axis: layout.Vertical}.Layout(gtx,
+			layout.Rigid(labeledEditor(th, i18n.T("label_password_current"), &d.oldEd, "")),
+			layout.Rigid(labeledEditor(th, i18n.T("label_password_new"), &d.newEd, "")),
+			layout.Rigid(labeledEditor(th, i18n.T("label_password_confirm"), &d.confirmEd, "")),
+			layout.Rigid(layout.Spacer{Height: unit.Dp(4)}.Layout),
+			layout.Rigid(material.Caption(th, i18n.T("hint_password_empty_disables")).Layout),
+			layout.Rigid(errorLabel(th, d.errorMsg)),
+		)
+	}
+	return modalCard(gtx, th, i18n.T("dialog_change_password_title"),
+		i18n.T("btn_ok"), i18n.T("btn_cancel"),
+		&d.okBtn, &d.cancelBtn, body)
+}