Feature: Add and implement i18n support for theme settings in German and English, refactor AppSettings to DesignSettings, and improve settings UI structure.

finlog_app/app/assets/i18n/de.i18n.json

@@ -25,7 +25,14 @@
     "tooltipUserSettings": "Benutzer-Einstellungen",
     "tooltipCollapseRail": "Leiste verkleinern",
     "tooltipExpandRail": "Leiste erweitern",
-    "drawerSettings": "Einstellungen"
+    "drawerSettings": "Einstellungen",
+    "settings": {
+      "theme": {
+        "system": "System",
+        "light": "Hell",
+        "dark": "Dunkel"
+      }
+    }
   },
   "settings": {
     "title": "Einstellungen",

finlog_app/app/assets/i18n/en.i18n.json

@@ -25,7 +25,14 @@
     "tooltipUserSettings": "User Settings",
     "tooltipCollapseRail": "Collapse Rail",
     "tooltipExpandRail": "Expand Rail",
-    "drawerSettings": "Settings"
+    "drawerSettings": "Settings",
+    "settings": {
+      "theme": {
+        "system": "System",
+        "light": "Light",
+        "dark": "Dark"
+      }
+    }
   },
   "settings": {
     "title": "Settings",