encrypted file-based token storage with keyring abstraction

[Sreehari425]

so this PR adds an alternative to the system keyring for storing auth tokens since the keyring can be kinda unreliable on some setups

tokens are encrypted with AES-256-GCM and the key is derived via Argon2id so its not just a plaintext file chillin on disk. there's a password prompt on startup if the store is locked and a new Security tab in settings where you can swap backends set a password or nuke the store entirely

all the auth flows (microsoft, elyby, littleskin, yggdrasil) got refactored to go through a shared token_store abstraction instead of hitting the keyring directly which should make adding new backends way easier down the line
also accounts are filtered by backend so keyring accounts wont bleed into encrypted file mode and vice versa

new deps: aes-gcm argon2 base64 rand

[Sreehari425]

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