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>

public/oa-drive/app.js

@@ -5,7 +5,8 @@
 
   function getStoredTheme() {
     const stored = localStorage.getItem(THEME_STORAGE_KEY);
-    return stored === 'dark' ? 'dark' : 'light';
+    if (stored === 'dark' || stored === 'light') return stored;
+    return 'dark';
   }
 
   function applyTheme(theme) {