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/BeckenFormPage.tsx

@@ -44,8 +44,9 @@ export default function BeckenFormPage() {
       return
     }
     setErrors({})
-    const saved = await mutation.run({ name: form.name.trim(), notes: nn(form.notes) })
-    navigate(`/becken/${saved.id}`)
+    const result = await mutation.run({ name: form.name.trim(), notes: nn(form.notes) })
+    if (result.ok) navigate(`/becken/${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>