feat: improve auth UX with autofill/theme fixes and stronger password validation

[ibsule]

Summary

• Improve auth form UX by fixing browser autofill styling so inputs remain readable and theme-consistent in both light and dark mode.
• Prevent initial theme flash on page load by adding early dark-mode aware styling in frontend/index.html.
• Strengthen backend validation by enforcing password length requirements in both reset-password and create-user DTOs.
• Update README.md with a note that the project is under active development.