Ship the registered EVE client ID as the default so sign-in works with nothing set #57

Part of the launch (Paperclip DIS-183).

EveProfitManager/sso.py _client_id() raises when EVE_CLIENT_ID is unset, so a stranger who downloads the app cannot sign in. The app uses PKCE with no client secret, so the client ID is public and may ship in the app (#18 research notes).

Default client ID: 28c1d7859b4e49f6bb97a13441e8be7c (callback http://127.0.0.1:8765/callback, already registered).

Acceptance criteria

• With EVE_CLIENT_ID unset, _client_id() returns the built-in default and sign-in starts.
• With EVE_CLIENT_ID set (non-empty), it overrides the default.
• The "Set the EVE_CLIENT_ID…" error is gone; tests cover default and override.
• README / run-windows skill note that the env var is now optional (override only).
• Full test suite passes.