fix: register /platform/ as post-logout redirect URI, improve sidebar contrast

- Add /platform/ to SPA postLogoutRedirectUris in bootstrap (fixes #54)
- Use amber color + bold for active vendor sidebar items

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

ui/src/components/Layout.tsx

@@ -69,14 +69,16 @@ export function Layout() {
         >
           <div
             style={{ padding: '6px 12px 6px 36px', fontSize: 13, cursor: 'pointer',
-              color: isActive(location, '/vendor/tenants') ? 'var(--text-primary)' : 'var(--text-muted)' }}
+              fontWeight: isActive(location, '/vendor/tenants') ? 600 : 400,
+              color: isActive(location, '/vendor/tenants') ? 'var(--amber)' : 'var(--text-muted)' }}
             onClick={() => navigate('/vendor/tenants')}
           >
             Tenants
           </div>
           <div
             style={{ padding: '6px 12px 6px 36px', fontSize: 13, cursor: 'pointer',
-              color: isActive(location, '/vendor/audit') ? 'var(--text-primary)' : 'var(--text-muted)' }}
+              fontWeight: isActive(location, '/vendor/audit') ? 600 : 400,
+              color: isActive(location, '/vendor/audit') ? 'var(--amber)' : 'var(--text-muted)' }}
             onClick={() => navigate('/vendor/audit')}
           >
             Audit Log