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[] = [];
 

src/lib/utils/theme-preview.ts

@@ -52,6 +52,7 @@ const appliedElements: { el: HTMLElement; prop: string }[] = [];
  * Only applies fields for the given mode (light or dark).
  */
 export function applyThemePreview(draft: TenantThemeData, mode: 'light' | 'dark') {
+	if (typeof document === 'undefined') return;
 	clearThemePreview();
 
 	const fieldMap = mode === 'light' ? LIGHT_FIELD_MAP : DARK_FIELD_MAP;
@@ -118,6 +119,7 @@ export function applyThemePreview(draft: TenantThemeData, mode: 'light' | 'dark'
  * Removes all inline CSS var overrides applied by applyThemePreview.
  */
 export function clearThemePreview() {
+	if (typeof document === 'undefined') return;
 	const el = document.documentElement;
 	for (const v of appliedVars) {
 		el.style.removeProperty(v);