test(cli): every test asks the platform where the data root is

The four other test files that spelled out `.local/share/wanderbooks`
(setup, books_home, format_upgrade, backups) use `default_root` and
`platform_data_dir`, so they hold on macOS as on Linux.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-cli/tests/backups.rs

@@ -44,7 +44,7 @@ impl Home {
     }
 
     fn root(&self) -> PathBuf {
-        self.dir.path().join(".local/share/wanderbooks")
+        ledger_store::root::default_root(self.dir.path())
     }
 
     fn book_dir(&self, book: &str) -> PathBuf {