fix: initialize darkMode in Dev.svelte to avoid props_invalid_value error

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/lib/components/Dev.svelte

@@ -109,7 +109,7 @@
 	}
 
 	let darkModeToggle: DarkModeToggle | undefined = $state()
-	let darkMode: boolean | undefined = $state(undefined)
+	let darkMode: boolean = $state(document.documentElement.classList.contains('dark'))
 	let modeInitialized = $state(false)
 	function initializeMode() {
 		modeInitialized = true