chore: ignore data/backend/

The Stockholm bridge persists its native-bridge state into
`data/backend/state/native-state.json` (per pkg/service/stockholm/handler.go,
which mkdir-p's `<workspaceRoot>/backend/state/`). The directory accumulates
per-session state — auth tokens, guids, device caches — that's not
meant to be tracked alongside the source.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

data/.gitignore

@@ -1,4 +1,5 @@
 accounts/
+backend/
 certs/
 default/
 dns/