feat: validate slug uniqueness during onboarding

Add GET /api/onboarding/slug-available endpoint to check if a slug is
already taken. Frontend checks availability with 400ms debounce as the
user types and shows inline feedback. Submit button disabled when slug
is taken. POST /api/onboarding/tenant now returns 409 instead of 500
for duplicate slugs.

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

ui/src/pages/OnboardingPage.tsx

@@ -1,4 +1,4 @@
-import { useState } from 'react';
+import { useState, useEffect, useRef } from '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';
@@ -16,9 +16,32 @@ export function OnboardingPage() {
   const [name, setName] = useState('');
   const [loading, setLoading] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  const [slugAvailable, setSlugAvailable] = useState<boolean | null>(null);
+  const [checkingSlug, setCheckingSlug] = useState(false);
+  const debounceRef = useRef<ReturnType<typeof setTimeout>>(undefined);
 
   const slug = toSlug(name);
 
+  useEffect(() => {
+    setSlugAvailable(null);
+    if (!slug) return;
+
+    setCheckingSlug(true);
+    clearTimeout(debounceRef.current);
+    debounceRef.current = setTimeout(async () => {
+      try {
+        const res = await api.get<{ available: boolean }>(`/onboarding/slug-available?slug=${encodeURIComponent(slug)}`);
+        setSlugAvailable(res.available);
+      } catch {
+        setSlugAvailable(null);
+      } finally {
+        setCheckingSlug(false);
+      }
+    }, 400);
+
+    return () => clearTimeout(debounceRef.current);
+  }, [slug]);
+
   async function handleSubmit(e: React.FormEvent) {
     e.preventDefault();
     setError(null);
@@ -29,7 +52,12 @@ export function OnboardingPage() {
       // picks up the new org membership and scopes on the next token refresh.
       window.location.href = '/platform/';
     } catch (err) {
-      setError(err instanceof Error ? err.message : 'Failed to create tenant');
+      const msg = err instanceof Error ? err.message : String(err);
+      if (msg.includes('409')) {
+        setError('This organization name is already taken. Try a different organization name.');
+      } else {
+        setError(msg || 'Failed to create tenant');
+      }
       setLoading(false);
     }
   }
@@ -67,7 +95,13 @@ export function OnboardingPage() {
               </FormField>
 
               <FormField label="URL slug" htmlFor="onboard-slug" required
-                hint="Auto-generated from name. Appears in your dashboard URL."
+                hint={
+                  !slug ? 'Auto-generated from name. Appears in your dashboard URL.'
+                    : checkingSlug ? 'Checking availability...'
+                    : slugAvailable === true ? 'Available'
+                    : 'Auto-generated from name. Appears in your dashboard URL.'
+                }
+                error={slugAvailable === false ? 'This organization name is already taken. Try a different name.' : undefined}
               >
                 <Input
                   id="onboard-slug"
@@ -81,7 +115,7 @@ export function OnboardingPage() {
                 variant="primary"
                 type="submit"
                 loading={loading}
-                disabled={loading || !name || !slug}
+                disabled={loading || !name || !slug || slugAvailable === false}
                 className={styles.submit}
               >
                 {loading ? 'Creating...' : 'Create workspace'}