Add configurable api_url for proxy support

[kenliu]

Closes #290

Changes

• Added BaseURL field to Config struct in lib/todoist.go
• Added baseURL() helper on Client that falls back to the hardcoded Server constant when BaseURL is not set
• Both doApi and doRestApi now use c.baseURL() instead of Server directly
• Reads api_url from config/env in main.go and passes it to Config

Usage

In ~/.config/todoist/config.json:

{
  "token": "your-token",
  "api_url": "https://your-proxy.example.com/api/v1/"
}

Or via environment variable:

TODOIST_API_URL=https://your-proxy.example.com/api/v1/ todoist list

No change in behavior when api_url is not set.