HOTFIX: useMutation.run returns MutationOutcome (no throw); sweep all 7 call sites + README convention

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

gerbil-manager-web/src/pages/WurfFormPage.tsx

@@ -116,13 +116,9 @@ export default function WurfFormPage() {
       expectedGoHomeDate: nn(form.expectedGoHomeDate),
       notes: nn(form.notes),
     }
-    try {
-      const saved = await mutation.run(body)
-      navigate(`/wuerfe/${saved.id}`)
-    } catch {
-      // Error surfaced via mutation.error; swallow to avoid an unhandled
-      // rejection escaping this async submit handler.
-    }
+    const result = await mutation.run(body)
+    if (result.ok) navigate(`/wuerfe/${result.value.id}`)
+    // On failure mutation.error drives the inline alert; run() never throws.
   }
 
   if (isEdit && existing.loading) return <p className="muted">{de.common.loading}</p>