fix: replace localhost help_url defaults with production URL

[Copilot]

AppConfig.help_url fell back to http://localhost:3000/help when the YAML key was absent or the dataclass was constructed without an explicit value — silently breaking the help button (webbrowser.open) for end users.

Changes

• src/voxkit/config/app_config.py — corrected both the dataclass field default and the from_yaml fallback:

  # before
  help_url: str = "http://localhost:3000/help"
  help_url=data.get("help_url", "http://localhost:3000/help"),
  
  # after
  help_url: str = "https://voxkit-web.vercel.app/help"
  help_url=data.get("help_url", "https://voxkit-web.vercel.app/help"),

• src/voxkit/config.py — updated the stale legacy HELP_URL constant to the production URL
• tests/config/test_app_config.py — updated two default-value assertions to match the corrected URL

Testing

• Test testable logic
• Edge cases addressed

Testing Notes: Updated assertions in test_dataclass_defaults and test_from_yaml_with_defaults now validate the correct production fallback URL.

Documentation

• README updated (if applicable)
• Code comments added/updated
• User-facing documentation updated (if applicable)

Documentation Notes: No documentation changes required; all YAML configs already use the correct production URL.

Checklist

• Code follows project conventions
• No breaking changes (or documented if intentional)
• All CI checks pass