Create a Tauri v2 plugin that uses EncryptedSharedPreferences (Android Keystore)
on Android and the system keychain (keyring crate) on desktop. This replaces
the direct onyx-core keyring calls in the Tauri app, which failed on Android
because keyring-storage was feature-gated to desktop only.
- New plugin crate at apps/tauri/tauri-plugin-credentials/ with Kotlin Android
code and Rust desktop fallback
- Update all Tauri credential commands to use the plugin API
- Add security-crypto dependency for Android and ProGuard rule for Tink
- Remove onyx-core/keyring-storage dependency from Tauri app features
