fix(ui): extract meaningful error messages from API responses

Introduces ApiError class in client.ts that parses Spring Boot error
bodies to extract human-readable messages (message, error, detail fields).
Adds errorMessage() helper used by all toast descriptions instead of
raw String(err) which dumped JSON blobs to the user.

Affected: all 10 page components that display error toasts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/OnboardingPage.tsx

@@ -2,7 +2,7 @@ import { useState, useEffect, useRef } from 'react';
 import { useLogto } from '@logto/react';
 import { Card, Input, Button, FormField, Alert } from '@cameleer/design-system';
 import cameleerLogo from '@cameleer/design-system/assets/cameleer-logo.svg';
-import { api } from '../api/client';
+import { api, errorMessage } from '../api/client';
 import { toSlug } from '../utils/slug';
 import styles from './OnboardingPage.module.css';
 
@@ -56,7 +56,7 @@ export function OnboardingPage() {
       // auto-approves and redirects back with fresh tokens.
       await signIn(`${window.location.origin}/platform/callback`);
     } catch (err) {
-      const msg = err instanceof Error ? err.message : String(err);
+      const msg = err instanceof Error ? err.message : errorMessage(err);
       if (msg.includes('409')) {
         setError('This organization name is already taken. Try a different organization name.');
       } else {