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/BeckenDetailPage.tsx

@@ -42,16 +42,14 @@ export default function BeckenDetailPage() {
   async function onDelete() {
     if (!window.confirm(t.delete.confirmMessage)) return
     setDeleteError(null)
-    try {
-      await removal.run()
+    const result = await removal.run()
+    if (result.ok) {
       navigate('/becken')
-    } catch (err) {
+    } else if (result.cause instanceof ApiError && result.cause.status === 409) {
       // Backend meldet Konflikt, wenn noch Tiere im Becken wohnen.
-      if (err instanceof ApiError && err.status === 409) {
-        setDeleteError(t.delete.conflict)
-      } else {
-        setDeleteError(removal.error ?? de.api.errors.unknown)
-      }
+      setDeleteError(t.delete.conflict)
+    } else {
+      setDeleteError(result.error)
     }
   }