feat: add user signup page with form validation

[omthorat2004]

Closes #21

Summary

Add a signup page to allow users to register via UI instead of only through the API.

Changes

• Add signup template (templates/signup.html) with form fields: name, surname, email, username, password
• Add GET /signup route and GetUserSignupService to render the signup page
• Add POST /signup route and SignupCookieSessionUserService to handle form submission
• Add client-side validation (HTML patterns) and server-side validation (regex + Pydantic EmailStr)
• Handle validation errors gracefully with redirect instead of raw 422 responses
• Handle duplicate user (IntegrityError) with user-friendly error message
• Add navigation links between sign in and sign up pages
• Add signup_success message on auth page after successful registration