Function select_backend 

pub fn select_backend(
    fallback_dir: impl Into<PathBuf>,
    fallback_passphrase: &str,
) -> (Box<dyn KeychainBackend + Send + Sync>, bool)

Select the best available keychain backend at runtime.

Probes the OS keychain by writing and deleting a test entry under the "brows3r" service. Returns a KeyringBackend on success; falls back to a FileBackendWithPassphrase otherwise.

Notes on fallback_passphrase

The passphrase prompt UX lands in task 18 (Credential Manager UI). At this stage the caller supplies the passphrase string directly.

OCP note

Future selection logic (env-var override, settings flag, 1Password integration) extends only this function without breaking call sites.