feat(ui): 接入 vendor 真实图标库 + 图标选择器

- assets/icons/ 共 80+ 张真实品牌图标嵌入 UI 包
- entry 新增 IconKey 字段，持久化到 config.Entry.IconName
- iconPickerDialog 网格布局图标选择器
- Add 完成后自动弹出图标选择器，Skip 则用 vendor 占位图
- ⋯ 菜单新增 "Change icon..." 项
- vendorIconFor 解析顺序：catalog → vendor 占位 → qr 兜底

internal/ui/entry_ring.go

@@ -26,11 +26,16 @@ func entryProgressRing(gtx layout.Context, en *entry) layout.Dimensions {
 	}
 
 	vendor := en.Auth.Name()
+	// Prefer the user-chosen brand icon over the vendor placeholder.
+	iconKey := vendor
+	if en.IconKey != "" {
+		iconKey = en.IconKey
+	}
 
 	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)
+		return drawCenteredIcon(gtx, iconKey, ringDp, ringDp)
 	}
 
 	period := totpPeriod(en.Auth)
@@ -64,7 +69,7 @@ func entryProgressRing(gtx layout.Context, en *entry) layout.Dimensions {
 	iconGtx.Constraints.Min = image.Pt(iconPx, iconPx)
 	iconGtx.Constraints.Max = image.Pt(iconPx, iconPx)
 	macro := op.Record(gtx.Ops)
-	paintIcon(iconGtx, vendor)
+	paintIcon(iconGtx, iconKey)
 	call := macro.Stop()
 	offset := image.Pt(
 		(ringDims.Size.X-iconPx)/2,