feat: store sensitive info in keychain

[kevcube]

Stores sensitive information in keychain. Tested on macOS only.

updated to use new bun.secrets API

replaces get() set() all() delete() functions in auth to use keychain instead of file.

pros:

• increases security

cons:

• removes text-editability of API keys
• breaking change to UX, requiring either automatic migrations of secrets into keychain, or users must re-auth to everything.

[kevcube]

this was oneshotted with opencode and gpt-5. if any changes are desired lmk

[kevcube]

@thdxr looking for review here. I have a patch ready to perform automatic migration-into-keychain every time a user accesses a secret.