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/pages/GerbilFormPage.tsx

@@ -166,8 +166,13 @@ export default function GerbilFormPage() {
       genotype: nn(form.genotype),
       notes: nn(form.notes),
     }
-    const saved = await mutation.run(body)
-    navigate(`/rennmaeuse/${saved.id}`)
+    try {
+      const saved = await mutation.run(body)
+      navigate(`/rennmaeuse/${saved.id}`)
+    } catch {
+      // Error is surfaced via mutation.error; swallow so the rejected promise
+      // from this async submit handler doesn't become an unhandled rejection.
+    }
   }
 
   if (isEdit && existing.loading) return <p className="muted">{de.common.loading}</p>