Patch 1.5.3, Fixes theme sync issue on first startup

Addresses a bug where the email viewer's dark mode setting was not correctly synchronized with the selected theme on the first application launch.

This ensures a consistent user experience regarding light/dark mode across the application and email viewer.

frontend/src/routes/(app)/settings/+page.svelte

@@ -211,6 +211,16 @@
     })();
   });
 
+  // Sync theme with email viewer dark mode
+  let previousTheme = $state<string | undefined>(undefined);
+  $effect(() => {
+    if (!browser) return;
+    if (previousTheme !== undefined && form.theme !== previousTheme) {
+      form.useDarkEmailViewer = form.theme === "dark";
+    }
+    previousTheme = form.theme;
+  });
+
   async function exportSettings() {
     try {
       const settingsJSON = JSON.stringify(form, null, 2);