Rename the darker tone to AMOLED; file icons are always colored

- Tone: the "darker" option is labelled "AMOLED" in all 7 locales — it is
  the true-black AMOLED palette. The persisted value stays "darker", so no
  settings migration.
- File icons: the Plain/Multi option is gone and the per-category glyphs +
  colors are always on. Removed the iconColors setting (type, default,
  signal, setter, options row, strings) and the plain branch in
  file-icons.ts. A stale "iconColors" key in settings.json is carried
  harmlessly and dropped on the next save.

Co-Authored-By: Claude <noreply@anthropic.com>

src/shared/i18n/ja.ts

@@ -35,7 +35,7 @@ export const ja = {
 		language: "言語",
 		system: "システム",
 		theme: { label: "テーマ", system: "システム", light: "ライト", dark: "ダーク" },
-		tone: { label: "トーン", dark: "ダーク", darker: "より暗く" },
+		tone: { label: "トーン", dark: "ダーク", darker: "AMOLED" },
 		accent: { label: "アクセント" },
 		accentNames: {
 			graphite: "グラファイト",
@@ -47,7 +47,6 @@ export const ja = {
 			green: "グリーン",
 			teal: "ティール",
 		},
-		fileIcons: { label: "ファイルアイコン", plain: "シンプル", multi: "カラー" },
 		interface: { label: "インターフェース", smaller: "縮小", larger: "拡大", reset: "100%に戻す" },
 		speedUnit: { label: "速度の単位" },
 		transfers: { label: "転送", single: "1件ずつ", multi: "複数" },