Add cluster dashboard command

[Davidonium]

Add cluster dashboard command

Adds qcloud cluster dashboard <cluster-id>, which opens a cluster's dashboard in your default browser.

# Open the dashboard in your browser
qcloud cluster dashboard 7b2ea926-724b-4de2-b73a-8675c42a6ebe

# Print the URL instead (headless / SSH sessions)
qcloud cluster dashboard 7b2ea926-724b-4de2-b73a-8675c42a6ebe --print-url

Highlights

• Verifies the cluster exists before opening, so you get a clear error instead of a browser tab pointing at a dead URL.
• Authentication is handled by your existing browser session: the command opens the Cloud UI, which redirects to the cluster dashboard.
• --print-url flag for headless or remote environments where no browser is available.
• Cluster ID tab-completion.

Supporting changes

• New --console-url global flag, QDRANT_CLOUD_CONSOLE_URL env var, and console_url config key (defaults to https://cloud.qdrant.io).
• State.OpenBrowser (backed by pkg/browser) with an injectable opener for testing.
• Regenerated command reference docs.