Split login/register into separate routes

[cfuehrmann]

Summary

• Split the combined /login page into separate /login and /register routes
• Extract shared getErrorMessage utility to src/lib/errors.ts
• Replace toggle buttons with Next.js Link components for navigation between routes

Why

The login page used conditional rendering (showRegistration state) to toggle between login and registration views. Separate routes are better:

• Deep linking — you can link directly to /register
• Browser history — back button works naturally between the two pages
• Simpler components — each page does one thing, no conditional state
• Analytics — distinct page views without custom event tracking

Changes

File	Change
src/lib/errors.ts	New — extracted getErrorMessage shared by both pages
src/app/(auth)/register/page.tsx	New — registration page with its own Suspense fallback
src/app/(auth)/login/page.tsx	Simplified — login only, removed all registration code
tests/e2e/auth.spec.ts	Updated — tests navigate directly to /register instead of clicking through /login

The "can toggle between login and registration views" test is replaced with "login page links to register and vice versa" which verifies the Link navigation between routes.

All checks pass: build, lint, format, knip, unit tests, 11/11 e2e tests.