revert(ui): drop welcome→login view transition
The View Transition API uses bitmap snapshots, which can't fake "the same
element" morphing across very different aspect ratios — corners and inset
shadow visibly distort and the cross-fade reads as an overlay. Removing it
in favour of a plain navigation with a small fade-in on the login form.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>