fix: Use dynamic redirect_uri for staging environment support

Changed Auth0 callback URL from hardcoded production domain to
window.location.origin, enabling login flow on staging.motovaultpro.com.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/core/auth/Auth0Provider.tsx

@@ -39,8 +39,8 @@ export const Auth0Provider: React.FC<Auth0ProviderProps> = ({ children }) => {
       domain={domain}
       clientId={clientId}
       authorizationParams={{
-        // Production domain; ensure mobile devices resolve this host during testing
-        redirect_uri: "https://motovaultpro.com/callback",
+        // Dynamic callback - supports staging (staging.motovaultpro.com) and production (motovaultpro.com)
+        redirect_uri: `${window.location.origin}/callback`,
         audience: audience,
         scope: 'openid profile email offline_access',
       }}