fix: handle Linux secret-service fallback for API keys

[tsubasakong]

Summary

• detect Linux Secret Service / DBus activation failures when saving the consolidated desktop secrets vault
• fall back to the Linux kernel keyring (keyutils) so API keys can still be saved in headless / Wayland sessions without an activatable Secret Service backend
• load previously saved fallback secrets on startup and clear the fallback entry once the normal keyring path succeeds again

Testing

• cargo check
• cargo test secret_storage_tests --bin world-monitor

Fixes #802

[vercel]

@tsubasakong is attempting to deploy a commit to the Elie Team on Vercel.

A member of the Team first needs to authorize it.