fix(keyring): update dependencies and improve error handling

[WeekendSuperhero]

Summary

• Updated keyring-core and platform native keyring dependencies to their latest versions.
• Enhanced macOS keychain initialization with probing and fallback to a file-based backend.
• Introduced a typed SecretError enum for better diagnostics of platform-specific errors.
• Refactored sorting across multiple modules to use sort_by_key with Reverse.

Changes

Keyring & Secrets

• Bumped keyring-core to 1.0, apple-native-keyring-store to 1.0 (with protected feature), windows-native-keyring-store to 1.1, and dbus-secret-service-keyring-store to 1.0.
• Implemented init_platform_keyring() in main.rs to handle platform-specific store setup and fallbacks.
• Replaced string-based errors with SecretError in secret.rs and added platform error code parsing for macOS (-25307, -25308, -34018).
• Added unit tests for all Secret backends (raw, command, keyring) and error classification.

Refactoring & Tooling

• Bumped rmcp to 1.7.
• Simplified descending sort logic in imap_client.rs and lib.rs using std::cmp::Reverse.
• Updated ci-check.sh to prefer cargo-nextest if available, falling back to cargo test.

Test plan

• Run ci-check.sh to confirm tests pass (using cargo-nextest if available).
• Verify macOS keychain initialization succeeds or gracefully falls back in headless/unsigned contexts.
• Confirm credential storage works on Windows and Linux.
• Ensure lists and flags continue to sort in descending order based on count.

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