fix: default theme to system, set data-theme attribute on DOM

- Default preference to 'system' so first-run users get OS preference detection
- Set data-theme attribute on documentElement alongside .dark class toggle

src/contexts/ThemeContext.tsx

@@ -25,7 +25,7 @@ function getStoredPreference(): ThemePreference {
 	} catch {
 		// Ignore storage errors
 	}
-	return "dark";
+	return "system";
 }
 
 function resolveTheme(pref: ThemePreference): ResolvedTheme {
@@ -39,6 +39,7 @@ function resolveTheme(pref: ThemePreference): ResolvedTheme {
 
 function applyThemeToDOM(theme: ResolvedTheme) {
 	const root = document.documentElement;
+	root.dataset.theme = theme;
 	if (theme === "dark") {
 		root.classList.add("dark");
 	} else {