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/de.json

@@ -100,10 +100,10 @@
   },
   "AppearanceSettings": {
     "sectionTitle": "Design-Erscheinungsbild",
-    "sectionDescription": "Wähle Hell, Dunkel oder System folgen. Einstellungen werden automatisch gespeichert.",
-    "themeMode": "Design-Modus",
-    "placeholder": "Design-Modus wählen",
-    "system": "System folgen",
+    "sectionDescription": "Wähle hell, dunkel oder Systemvorgabe. Einstellungen werden automatisch gespeichert.",
+    "themeMode": "Designmodus",
+    "placeholder": "Designmodus auswählen",
+    "system": "Systemvorgabe",
     "light": "Hell",
     "dark": "Dunkel",
     "currentTheme": "Aktuell wirksames Design: {theme}",