Fix branding page 500 on hard refresh: guard document access during SSR

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

src/lib/utils/theme-persist.ts

@@ -42,6 +42,7 @@ function hexToRgba(hex: string, opacity: number): string {
  * so theme changes take effect immediately without a full page reload.
  */
 export function persistThemeCSS(theme: TenantThemeData) {
+	if (typeof document === 'undefined') return;
 	const lightVars: string[] = [];
 	const darkVars: string[] = [];