[Feature] Login screen redesign

[lobster40]

Summary

As a user I want a modern, responsive login screen so that signing in feels fast and works on any device.

Area

ui

Acceptance criteria

• New login layout matches the approved design
• Works on mobile, tablet, desktop breakpoints
• Client-side + server-side validation with clear errors

Gates

• Specifications accepted
• Architecture design passed
• Development done
• Testing done
• Release notes accepted

Product release note

User-visible change: Redesigned login screen.
Breaking change: No
Migration required: No
Docs required: No

[github-actions]

🔧 Reconciled: Specifications set to Passed to match the checklist (the field had drifted).

[github-actions]

🔧 Reconciled: Architecture design set to Passed to match the checklist (the field had drifted).