feat(api,web): Restrict registration to waitlist emails and revamp auth UX

[shettydev]

Summary

• enforce waitlist-only signup at the API registration endpoint before user creation
• keep signup email normalized in controller and return a clear forbidden error when email is not waitlisted
• refresh the auth page UI/UX with a more structured, centered card and clearer trust/context blocks
• add explicit signup error guidance in UI with a direct link back to the waitlist section

Implementation details

• auth module now imports waitlist module so auth controller can query waitlist service
• auth controller register endpoint checks waitlist membership via waitlist service
• web signup form surfaces waitlist restriction errors and includes a join waitlist CTA
• auth page layout was revamped while preserving existing auth flows and routing

Validation

• bun --cwd packages/mukti-web test -- --runInBand (all suites pass)
• bun --cwd packages/mukti-api build
• bun --cwd packages/mukti-web test -- sign-up-form.test.tsx --runInBand