Support custom config file location

[yimsk]

Summary

Allow users to specify a custom config file path instead of the default ~/.config/claws/config.yaml.

Motivation

• CI/CD environments may need project-specific configs
• Testing with different configurations
• Multi-environment setups (work/personal)
• Containerized deployments with mounted config files

Proposed Implementation

CLI Flag

claws --config /path/to/config.yaml
claws -c /path/to/custom-config.yaml

Environment Variable

CLAWS_CONFIG=/path/to/config.yaml claws

Precedence

1. --config / -c flag (highest)
2. CLAWS_CONFIG environment variable
3. ~/.config/claws/config.yaml (default)

Acceptance Criteria

• Add -c, --config <path> CLI flag
• Support CLAWS_CONFIG environment variable
• Validate file exists and is readable
• Update config.File() to use specified path
• Autosave writes to the specified path (not default)
• Update README with documentation
• Add tests for custom config path