feat(security): enable system keyring for credential storage

Enable async-secret-service + crypto-rust features on the keyring
crate, activating GNOME Keyring and KDE Wallet support. The keyring
integration code was already written but the crate was compiled
without any platform backend, so it silently failed every time.

OAuth tokens will now be stored in the system keyring when available,
with encrypted file as automatic fallback.

Added org.freedesktop.secrets to Flatpak finish-args for sandbox
keyring access.

Fixes #215

cc

src-tauri/Cargo.toml

@@ -130,8 +130,8 @@ urlencoding = "2"
 # Environment variables
 dotenvy = "0.15"
 
-# Secure credential storage (system keyring)
-keyring = "3"
+# Secure credential storage (system keyring: GNOME Keyring, KDE Wallet, macOS Keychain)
+keyring = { version = "3", features = ["async-secret-service", "crypto-rust"] }
 
 # Clipboard
 tauri-plugin-clipboard-manager = "2"