feat(demo): deeply nested sample note to try out zooming

"Hausrenovierung 2027" in Privat goes down seven levels (floor → room →
topic → appointment → variants → details) with tasks and hashtags mixed
in, so zooming into a branch and back via the breadcrumb shows something
realistic.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/docsync_test.go

@@ -403,7 +403,7 @@ func TestDemoAccountsGetSampleDataAndShares(t *testing.T) {
 	}
 	var docs int
 	_ = s.db.QueryRow(`SELECT COUNT(*) FROM documents WHERE user_id = ?`, admin.ID).Scan(&docs)
-	if docs != 3 {
+	if docs != 4 {
 		t.Fatalf("restart seeded again: %d notes", docs)
 	}
 }