feat: add setting to disable tag-color row tint in message list

Message-list rows are tinted with the first tag's color, which becomes
overwhelming when a label applies to most messages (for example
per-account labels). Add a `tintListRowsByTag` setting (default true, so
current behavior is unchanged) with a toggle in Settings beside
"Colorful Sidebar Icons". When off, rows are not tinted; tag dots and
chips still show the color. Gated in both list renderers
(email-list-item and thread-list-item).

locales/ja/common.json

@@ -938,6 +938,10 @@
         "label": "カラフルなサイドバーアイコン",
         "description": "フォルダーとタグのアイコンを種類別に色分けします（受信トレイは青、迷惑メールは赤、送信済みは緑など）。モノクロのサイドバーにするには無効にしてください。"
       },
+      "tint_list_rows": {
+        "label": "Tint List Rows by Tag Color",
+        "description": "Shade each message row with its first tag color. Disable to keep rows plain; tag dots and chips still show the color."
+      },
       "show_folder_total_count": {
         "label": "メッセージの総数を表示",
         "description": "フォルダーやタグの横に、未読数に加えてメッセージの総数を表示します。未読数のみを表示するには無効にします。"