fix: prevent white flash by initializing dark mode from system preference (#846)

Initialize dark mode state during app startup using system preferences and
stored user settings to prevent momentary white flash on page load.
Previously, dark mode was always initialized as false before checking preferences.

Author: T0biii

website/src/AppState.ts

@@ -13,7 +13,10 @@ class GlobalAppState {
       loadingError: observable,
     });
 
-    this.darkMode = false;
+    const prefersDarkMode = window.matchMedia('(prefers-color-scheme: dark)').matches;
+    const storedDarkMode = localStorage.getItem('customDarkMode');
+    
+    this.darkMode = storedDarkMode !== null ? JSON.parse(storedDarkMode) : prefersDarkMode;
   }
 
   setDarkMode(darkMode: boolean) {