fix: report login flow completion and errors

[juliusknorr]

What changed

• Pass the login UI dismissal callback through to the polling flow and invoke it after successful account creation.
• Treat HTTP 404 from the Login Flow v2 polling endpoint as the expected pending state.
• Stop retrying terminal polling failures and present them in a Login Failed alert.
• Log Login Flow v2 startup and polling failures with useful error details.
• Treat cancellation without an active polling task as an expected condition.

Why

The host app discarded the dismissal callback supplied by ServerAddressView, so a successful login was not explicitly reported back to the login UI. Polling also collapsed every unsuccessful response into a pending state, causing terminal server or network errors to retry indefinitely without visible feedback.

Impact

Successful browser-based login now closes the login UI and proceeds into the app. Expected pending responses remain silent, while genuine polling failures stop the flow and give the user an actionable error alert.

Validation

• Built the iOCNotes scheme successfully for an iPhone 17 Pro simulator running iOS 27.0.
• Installed and launched the resulting app in the simulator.
• Confirmed the app process remained running without a launch failure.