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/GerbilFormPage.tsx

@@ -166,13 +166,9 @@ export default function GerbilFormPage() {
       genotype: nn(form.genotype),
       notes: nn(form.notes),
     }
-    try {
-      const saved = await mutation.run(body)
-      navigate(`/rennmaeuse/${saved.id}`)
-    } catch {
-      // Error is surfaced via mutation.error; swallow so the rejected promise
-      // from this async submit handler doesn't become an unhandled rejection.
-    }
+    const result = await mutation.run(body)
+    if (result.ok) navigate(`/rennmaeuse/${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>