BUG: Keyring operations silently ignore errors

[andrinoff]

Describe the bug

In config/config.go lines 357-360 and 365-366, keyring operations suppress errors with _:

_ = keyring.Set(keyringServiceName, acc.Email, acc.Password)
_ = keyring.Delete(keyringServiceName, acc.Email)

If keyring unavailable or fails, passwords silently fail to persist with no warning/fallback.

To reproduce

1. Run matcha on system without keyring support
2. Save password - keyring.Set fails silently
3. Restart matcha - password lost, no indication why
4. User confused about missing credentials

Expected behavior

Log keyring errors as warnings:

if err := keyring.Set(keyringServiceName, acc.Email, acc.Password); err != nil {
    log.Printf("Warning: failed to store password in keyring: %v", err)
}

Or provide fallback mechanism for systems without keyring.

Screenshots

N/A

Additional context

• File: config/config.go
• Lines: 357-360, 365-366
• Severity: Medium - silent credential loss
• Fix complexity: Easy - add logging/warning