FEAT-3: fix unhandled ApiError rejections (catch in form submits + dev unhandledrejection guard, ApiError carries path)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

gerbil-manager-web/src/main.tsx

@@ -2,6 +2,9 @@ import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
 import './index.css'
 import App from './App.tsx'
+import { installUnhandledRejectionGuard } from './dev/unhandledRejectionGuard'
+
+installUnhandledRejectionGuard()
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>