fix(app-iced): 复审 r1 的四条 p2 + 三条小事（M250）

- p2-1 删死状态：`Accent::scheme` 字段 / `scheme()` 取值口 / `set_flavor` 重派生全仓零读者，
  且注释谎称「Ui 的唯一入口」。色值 `color` 与深浅无关，`Ui` 由 `rebuild_ui` 重算，
  所以整条删掉，`new/set/reset` 也不再收 `flavor`
- p2-2 规则只定义一次：新增 `theme::tokens(flavor, accent, scheme, families) -> (Ui, Colors)`,
  `App::rebuild_ui` 与出图 harness 都调它（harness 传 scheme=None，如实说明不覆盖 D38 分支）；
  README 改成「同一条规则、不是同一个调用点」
- p2-3 逐键不闪红：`Accent` 区分「显示中的错」与 `pending`。只有**不可能再变合法**（非 hex / 超 6 位）
  才立刻亮；位数不足先挂 pending，等 `commit()`（点到别的外观控件）或配置读回才亮
- p2-4 订正 `app/state.rs:151` 那句反话（强调色现在是 UI 写入字段）
- 小事：hint 文案改「中性表面与错误色不随强调色变」；`Digit{ch}` 里的不可见字符显示成 U+XXXX；
  8 颗色块补 tooltip（hex 原文，Overlay 底）

判据：新用例 `accent_errors_only_appear_when_they_are_actionable` 覆盖「输入中不亮 / 提交后亮 /
配置读回立刻亮」三态；gallery 出图逐字节与重构前相同（规则搬迁没改像素）。

crates/i18n/src/catalog/golden/tail2.rs

@@ -236,7 +236,7 @@ pub(super) const ROWS: &[(&str, &str, &str)] = &[
     ("SettingsAppearanceAccent", "Accent colour", "强调色"),
     ("SettingsAppearanceAccentPresets", "Presets", "预设"),
     ("SettingsAppearanceAccentFactory", "Factory", "出厂"),
-    ("SettingsAppearanceAccentHint", "Empty = the factory accent from the brand board. A custom accent replaces primary / secondary / tertiary only; neutral surfaces and error keep the baseline.", "空 = 出厂品牌板的强调色。自定义强调色只替换 primary / secondary / tertiary 三族；中性表面与 error 保持基线。"),
+    ("SettingsAppearanceAccentHint", "Empty = the factory accent from the brand board. A custom accent replaces primary / secondary / tertiary only; neutral surfaces and error colours do not follow it.", "空 = 出厂品牌板的强调色。自定义强调色只替换 primary / secondary / tertiary 三族；中性表面与错误色不随强调色变。"),
     ("SettingsAppearanceAccentShort", "Accent needs 6 hex digits (RRGGBB); got {digits}.", "强调色要 6 位十六进制（RRGGBB），现在 {digits} 位。"),
     ("SettingsAppearanceAccentLong", "Accent has {digits} digits; a colour takes exactly 6.", "强调色有 {digits} 位，一个颜色只要 6 位。"),
     ("SettingsAppearanceAccentDigit", "Accent contains a character that is not a hex digit: {ch}", "强调色里有非十六进制字符：{ch}"),

crates/i18n/src/catalog/settings/accent.rs

@@ -26,8 +26,8 @@ macro_rules! settings_accent_entries {
             SettingsAppearanceAccentPresets => ("Presets", "预设"),
             SettingsAppearanceAccentFactory => ("Factory", "出厂"),
             SettingsAppearanceAccentHint => (
-                "Empty = the factory accent from the brand board. A custom accent replaces primary / secondary / tertiary only; neutral surfaces and error keep the baseline.",
-                "空 = 出厂品牌板的强调色。自定义强调色只替换 primary / secondary / tertiary 三族；中性表面与 error 保持基线。"
+                "Empty = the factory accent from the brand board. A custom accent replaces primary / secondary / tertiary only; neutral surfaces and error colours do not follow it.",
+                "空 = 出厂品牌板的强调色。自定义强调色只替换 primary / secondary / tertiary 三族；中性表面与错误色不随强调色变。"
             ),
             SettingsAppearanceAccentShort => (
                 "Accent needs 6 hex digits (RRGGBB); got {digits}.",