feat(portal): default to dark theme for new visitors

First-time and cleared-storage users see dark mode; saved light preference is unchanged. Inline boot script avoids a light flash before React loads.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/utils/theme.ts

@@ -4,7 +4,8 @@ export const THEME_STORAGE_KEY = 'memind-theme';
 
 export function getStoredTheme(): MemindTheme {
   const stored = localStorage.getItem(THEME_STORAGE_KEY);
-  return stored === 'dark' ? 'dark' : 'light';
+  if (stored === 'dark' || stored === 'light') return stored;
+  return 'dark';
 }
 
 export function applyTheme(theme: MemindTheme) {