Login previously left the app entirely for a separate browser task. Now uses
androidx.browser Custom Tabs, which overlays the app's own activity instead of
switching away, and returns automatically on redirect. Deliberately not a
WebView, per RFC 8252: an in-app WebView could read the Gitea login form's
credentials directly.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
