fix(auth): trim anonymous registration usernames

[aiirvizionz]

Summary

• Trim anonymous registration usernames before duplicate checks and insert
• Trim display names and fall back to the cleaned username when blank
• Reject whitespace-only usernames before touching auth/session validation

Why

Anonymous invite registration accepted raw username strings. A value like alice could be stored with leading/trailing spaces, and a whitespace-only username could get past the basic truthiness check. That makes account display and duplicate detection less reliable.

Validation

• node --check src/app/api/auth/register-anon/route.js
• node --check src/app/api/auth/register-anon/route.test.js
• git diff --check

Note: node_modules/.bin/vitest.cmd run src/app/api/auth/register-anon/route.test.js still fails before test collection because this repo's Vitest config loads @vitejs/plugin-react against a Vite build that does not export ./internal under the current Windows/Node runtime. The focused regression test is included for CI once the project runner resolves normally.