feat(ui): 首次启动加密引导 + 真窗口图标 (.syso)

首次启动:
- 配置文件不存在时弹欢迎对话框：选「启用密码保护」或「稍后再说」
- 选启用直接进 setPasswordDialog，OK 时 store.SetPassword 触发首次保存
- io/fs.ErrNotExist 检测新分支
- modalCard 抽出 modalShell 内核以便 welcomeDialog 自定义 footer

真窗口图标:
- tools/gen_ico 从 PNG 生成多分辨率 .ico（7 尺寸：16~256）
- 提交 rsrc_windows.syso（rsrc 通过 goproxy.cn 生成），go build 自动链入
- tray_windows.go 改用嵌入资源 ID=1，失败 fallback IDI_APPLICATION
- README 更新完整的图标生成流程

internal/ui/dialog.go

@@ -34,8 +34,29 @@ func modalCard(
 	cancelBtn *widget.Clickable,
 	body layout.Widget,
 ) layout.Dimensions {
-	fillBackground(gtx, activePalette.ScrimBg)
+	footer := func(gtx layout.Context) layout.Dimensions {
+		return layout.Flex{Axis: layout.Horizontal, Spacing: layout.SpaceStart}.Layout(gtx,
+			layout.Rigid(material.Button(th, cancelBtn, cancelLabel).Layout),
+			layout.Rigid(func(gtx layout.Context) layout.Dimensions {
+				return layout.Inset{Left: unit.Dp(8)}.Layout(gtx,
+					material.Button(th, okBtn, okLabel).Layout)
+			}),
+		)
+	}
+	return modalShell(gtx, th, title, body, footer)
+}
 
+// modalShell is the common backdrop + title + body + custom-footer
+// layout. Used by modalCard for the standard OK/Cancel footer and by
+// dialogs (e.g. welcomeDialog) that need a custom action bar.
+func modalShell(
+	gtx layout.Context,
+	th *material.Theme,
+	title string,
+	body layout.Widget,
+	footer layout.Widget,
+) layout.Dimensions {
+	fillBackground(gtx, activePalette.ScrimBg)
 	return layout.Center.Layout(gtx, func(gtx layout.Context) layout.Dimensions {
 		gtx.Constraints.Max.X = gtx.Dp(420)
 		return widget.Border{
@@ -50,15 +71,7 @@ func modalCard(
 					layout.Rigid(layout.Spacer{Height: unit.Dp(12)}.Layout),
 					layout.Rigid(body),
 					layout.Rigid(layout.Spacer{Height: unit.Dp(12)}.Layout),
-					layout.Rigid(func(gtx layout.Context) layout.Dimensions {
-						return layout.Flex{Axis: layout.Horizontal, Spacing: layout.SpaceStart}.Layout(gtx,
-							layout.Rigid(material.Button(th, cancelBtn, cancelLabel).Layout),
-							layout.Rigid(func(gtx layout.Context) layout.Dimensions {
-								return layout.Inset{Left: unit.Dp(8)}.Layout(gtx,
-									material.Button(th, okBtn, okLabel).Layout)
-							}),
-						)
-					}),
+					layout.Rigid(footer),
 				)
 			})
 		})