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/convert.go

@@ -11,10 +11,11 @@ import (
 // in-memory authenticator plus its display name. The vendor string is
 // derived from the authenticator's Name() (which already returns
 // "google" / "microsoft" / "okta" / "hotp" / "battlenet" / "steam").
-func entryFromAuthenticator(name string, a authenticator.Authenticator, hotkey string) config.Entry {
+func entryFromAuthenticator(name string, a authenticator.Authenticator, hotkey, iconKey string) config.Entry {
 	return config.Entry{
 		Name:      name,
 		Vendor:    a.Name(),
+		IconName:  iconKey,
 		SecretRaw: a.SecretData(),
 		Hotkey:    hotkey,
 	}