tailwind v2 migration: step-zero prerequisites (#610)

- Fix flashesV2.ejs missing comma that silently dropped error flashes
- Port form-state preservation from stale root signupV2.ejs into views/public/signupV2.ejs
- Define empty.ejs as a real bare-shell layout (DOCTYPE + Tailwind, no nav/footer)
- Remove stale root signupV2.ejs and teamV2.ejs drafts (views/public versions are canonical)

Prereq for the broader Tailwind V2 migration tracked in #609.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: kokonut121