[BUG] Go quota sidebar fails after Console migration #48

Describe the bug

The OpenCode Go quota section in the sidebar displays Unable to fetch Go quota even when the workspace and browser auth cookie are configured.

The plugin still queried the legacy /workspace/<workspace>/go HTML page. OpenCode now redirects that route to the Console login flow, so the plugin received an empty/login page and could not parse quota data.

To Reproduce

1. Configure OPENCODE_GO_WORKSPACE_ID and OPENCODE_GO_AUTH_COOKIE.
2. Use an opencode-go model in OpenCode.
3. Open the sidebar.
4. Observe Unable to fetch Go quota or the previous all-zero warning.

Expected behavior

The sidebar should display rolling, weekly, and monthly Go quota usage using the current OpenCode Go usage API, with a clear authentication/setup message when credentials are invalid or unavailable.

Media

Not applicable.

Additional context

The current usage endpoint is GET https://opencode.ai/zen/go/v1/usage with a Bearer Console API key. The legacy workspace-page scraper remains only as a fallback for cookie-based setups.