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/dev/unhandledRejectionGuard.ts

@@ -0,0 +1,25 @@
+import { ApiError } from '../api/client'
+
+/**
+ * Dev-only safety net for unhandled promise rejections.
+ *
+ * Call sites should still handle their own errors (useApi error UI, try/catch
+ * around mutations). This guard exists so that any STRAY rejection — typically
+ * an ApiError from an endpoint that hasn't shipped yet — is logged with its
+ * failing path instead of spamming the console with "Uncaught (in promise)".
+ * It surfaces regressions (which endpoint, which status) without turning the
+ * page into a dead end. No-op in production builds.
+ */
+export function installUnhandledRejectionGuard(): void {
+  if (!import.meta.env.DEV) return
+  window.addEventListener('unhandledrejection', (event) => {
+    const reason = event.reason
+    if (reason instanceof ApiError) {
+      const where = reason.path ? ` [${reason.path}]` : ''
+      console.warn(`[API] Unbehandelte Ablehnung${where} (${reason.status ?? 'Netzwerk'}): ${reason.message}`)
+      event.preventDefault()
+    } else {
+      console.warn('[Unbehandelte Promise-Ablehnung]', reason)
+    }
+  })
+}