feat(ui): vendor 图标系统 + 应用图标占位

- 新增 tools/gen_icons 生成 7 张 vendor 占位 PNG + 256x256 应用图标
- internal/ui/vendor_icons.go 用 //go:embed 加载并按 Authenticator.Name() 查表
- entry 行圆环中心叠 16dp vendor 图标，HOTP 直接用图标占位
- 图标为通用色块 + 首字母，无第三方商标；用户可按文件名替换为真实 logo
- assets/README 说明替换方式与 Windows .syso 嵌入步骤

internal/ui/entry_ring.go

@@ -1,23 +1,38 @@
 package ui
 
 import (
+	"image"
 	"time"
 
 	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/op/paint"
 	"gioui.org/unit"
+	"gioui.org/widget"
 
 	"git.wxccs.org/iceking2nd/winauth-go/internal/authenticator"
 )
 
-// entryProgressRing draws the per-row TOTP countdown ring. For HOTP
-// entries it returns a same-sized blank box so the rows still line up.
+// entryProgressRing draws the per-row TOTP countdown ring with the
+// vendor icon nested in the centre. For HOTP entries the ring is
+// omitted (no countdown to show) and the icon fills the same slot so
+// rows still line up.
 func entryProgressRing(gtx layout.Context, en *entry) layout.Dimensions {
-	const ringDp = 22
-	if en.Auth == nil || en.Auth.Name() == "hotp" {
-		return layout.Dimensions{Size: gtx.Constraints.Constrain(
-			layout.Spacer{Width: unit.Dp(ringDp), Height: unit.Dp(ringDp)}.Layout(gtx).Size,
-		)}
+	const ringDp = 24
+	const iconDp = 16
+
+	if en.Auth == nil {
+		return layout.Dimensions{Size: image.Pt(gtx.Dp(ringDp), gtx.Dp(ringDp))}
 	}
+
+	vendor := en.Auth.Name()
+
+	if vendor == "hotp" {
+		// HOTP has no period to visualise — just show the icon scaled
+		// to the full slot so rows still align.
+		return drawCenteredIcon(gtx, vendor, ringDp, ringDp)
+	}
+
 	period := totpPeriod(en.Auth)
 	if period <= 0 {
 		period = authenticator.DefaultPeriod
@@ -33,13 +48,60 @@ func entryProgressRing(gtx layout.Context, en *entry) layout.Dimensions {
 	}
 	bg := activePalette.RingBg
 
-	return progressRing{
+	ringDims := progressRing{
 		Size:     unit.Dp(ringDp),
 		Stroke:   unit.Dp(2.5),
 		Progress: progress,
 		Color:    fg,
 		BgColor:  bg,
 	}.Layout(gtx)
+
+	// Overlay the vendor icon in the centre of the same slot. We do
+	// this by recording the icon op into a fixed-size context, then
+	// offsetting it so its centre matches the ring's centre.
+	iconPx := gtx.Dp(iconDp)
+	iconGtx := gtx
+	iconGtx.Constraints.Min = image.Pt(iconPx, iconPx)
+	iconGtx.Constraints.Max = image.Pt(iconPx, iconPx)
+	macro := op.Record(gtx.Ops)
+	paintIcon(iconGtx, vendor)
+	call := macro.Stop()
+	offset := image.Pt(
+		(ringDims.Size.X-iconPx)/2,
+		(ringDims.Size.Y-iconPx)/2,
+	)
+	st := op.Offset(offset).Push(gtx.Ops)
+	call.Add(gtx.Ops)
+	st.Pop()
+	return ringDims
+}
+
+// drawCenteredIcon paints a vendor icon scaled to fit a w×h slot. Used
+// for HOTP entries that have no ring around them.
+func drawCenteredIcon(gtx layout.Context, vendor string, wDp, hDp unit.Dp) layout.Dimensions {
+	wPx, hPx := gtx.Dp(wDp), gtx.Dp(hDp)
+	gtx.Constraints.Min = image.Pt(wPx, hPx)
+	gtx.Constraints.Max = image.Pt(wPx, hPx)
+	return paintIcon(gtx, vendor)
+}
+
+// paintIcon draws the vendor icon scaled to fit the current
+// constraints. Returns its layout dimensions so callers can compose it
+// inside a Flex. Returns a zero-sized layout if the icon is missing.
+func paintIcon(gtx layout.Context, vendor string) layout.Dimensions {
+	img := vendorIconFor(vendor)
+	if img == nil {
+		return layout.Dimensions{Size: gtx.Constraints.Min}
+	}
+	w := widget.Image{
+		Src:      paint.NewImageOp(img),
+		Fit:      widget.Contain,
+		Position: layout.Center,
+	}
+	// Match one source pixel to one device pixel scaled by Fit. We
+	// don't want device-DPI scaling on top of widget.Fit's own
+	// scaling, so leave Scale at its 1.0 default.
+	return w.Layout(gtx)
 }
 
 // totpPeriod extracts the configured period from any authenticator whose