Improve WellAI email verification

[Sarasaidin]

Summary

Improved the WellAI email verification flow with branded email content, verification pages, and static logo support.

Changes Made

• Updated the verification email subject to "Verify your WellAI account"
• Added WellAI branding and logo to the verification email
• Added a "Verify my email" button instead of displaying the long verification URL
• Added WellAI Privacy Notice to the email
• Updated the email footer with WellAI branding and copyright information
• Added a branded email verification success page
• Added a branded invalid/expired verification link page
• Added static file serving for the WellAI logo
• Confirmed unverified users cannot log in until their email is verified
• Fixed email footer character encoding

Testing

• Verified the WellAI logo is served successfully through the backend
• Tested email registration and verification flow
• Tested successful email verification page
• Tested invalid/expired verification link page
• Verified unverified users receive a 403 response when attempting to log in
• Ran git diff --check successfully