fix(i18n): restore pre-existing AppearanceSettings translations

Task 7 inadvertently overwrote 18 carefully-tuned translations across 7 languages
(ar/de/es/fr/ja/ko/pt) with the plan author's slightly different wording while
adding the new themeColor / zoomLevel / resetToDefaults keys. The new additions
remain intact; only the pre-existing keys are reverted to the project's original
translations (e.g. de "Designmodus" instead of "Design-Modus", ja
"システム追従" instead of "システムに従う"). Affects: sectionDescription,
themeMode, placeholder, system, currentTheme.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/i18n/messages/ja.json

@@ -100,13 +100,13 @@
   },
   "AppearanceSettings": {
     "sectionTitle": "テーマ外観",
-    "sectionDescription": "ライト、ダーク、またはシステムに従うを選択します。設定は自動的に保存されます。",
+    "sectionDescription": "ライト、ダーク、またはシステム追従を選択できます。設定は自動保存されます。",
     "themeMode": "テーマモード",
     "placeholder": "テーマモードを選択",
     "system": "システムに従う",
     "light": "ライト",
     "dark": "ダーク",
-    "currentTheme": "現在有効なテーマ：{theme}",
+    "currentTheme": "現在の有効テーマ: {theme}",
     "resolvedTheme": {
       "light": "ライト",
       "dark": "ダーク",