BUG: Missing validation for empty IMAP/SMTP server config

[andrinoff]

Describe the bug

In config/config.go lines 86 and 115, empty IMAPServer and SMTPServer values return empty strings without validation:

func (a *Account) GetIMAPServer() string {
    if a.IMAPServer == "" {
        return ""  // No error
    }
    return a.IMAPServer
}

Error caught later in fetcher/fetcher.go:236, but better to validate early at config load.

To reproduce

1. Create config with empty imap_server field
2. Start matcha
3. Connection fails with unclear error downstream
4. Should fail fast with clear validation error

Expected behavior

Validate config on load:

func (c *Config) Validate() error {
    for _, acc := range c.Accounts {
        if acc.IMAPServer == "" {
            return fmt.Errorf("account %q: imap_server is required", acc.Email)
        }
        if acc.SMTPServer == "" {
            return fmt.Errorf("account %q: smtp_server is required", acc.Email)
        }
    }
    return nil
}

Screenshots

N/A

Additional context

• File: config/config.go
• Lines: 86, 115
• Severity: Low-Medium - poor error UX
• Fix complexity: Easy - add validation function