Add saved login restore flow

[imissapixel]

I took the liberty of creating this feature for my own usage, and saw that there was a previous request for it here #32

Summary

Adds a saved-login restore flow so users do not need to complete Telegram login on every app launch.

Changes

• Adds cmd_auth_restore_session to restore an existing authorized local Telegram session.
• Adds a “Keep me logged in on this device” checkbox during login.
• Shows a brief “Checking Saved Session” state on startup.
• Reuses the existing local Telegram session when possible.
• Keeps explicit logout behavior unchanged.

Security Notes

• The API hash is not persisted for saved login.
• The app stores only remember_login and api_id.
• Existing stored api_hash is removed when auth state is saved.
• Session restore succeeds only if the local Telegram session is already authorized.

Verification

• cargo check
• cargo test
• npm run build
• Manual test: login persists after app restart.

[caamer20]

Smart! will merge later, thank you!