Implement config file support (~/.config/looprelay/config.yaml)

[rajfirke]

Description

Add optional YAML configuration at ~/.config/looprelay/config.yaml for:

• Default export format (html, md, json)
• Custom session discovery paths
• Theme settings

Implementation

• internal/config/config.go exists as a stub directory
• Use gopkg.in/yaml.v3 (already a dependency)
• Load config in root.go init()
• Works without config (zero-config by default)

Location

• internal/config/config.go (to create)
• internal/cli/root.go