Introduce a new token_store module that persists OAuth tokens in the OS
keychain (Credential Manager on Windows, Secret Service on Linux through
sync-secret-service + crypto-rust, both pure-Rust backends).
- Keychain service name matches the Tauri bundle identifier
(com.simpl.resultat) so credentials are scoped to the real app
identity.
- Transparent migration on first load: a legacy tokens.json is copied
into the keychain, then zeroed and unlinked before removal to reduce
refresh-token recoverability from unallocated disk blocks.
- Store-mode flag (keychain|file) persisted next to the auth dir.
After a successful keychain write the store refuses to silently
downgrade to the file fallback, so a subsequent failure forces
re-authentication instead of leaking plaintext.
- New get_token_store_mode command exposes the current mode to the
frontend so a settings banner can warn users running on the file
fallback.
- auth_commands.rs refactored: all tokens.json read/write/delete paths
go through token_store; check_subscription_status now uses
token_store::load().is_some() to trigger migration even when the
24h throttle would early-return.
Refs #66
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
