Changes related to login, register and verification

[martingaldeca]

Type of Change

• 🚀 New feature (non-breaking change which adds functionality)
• 🐛 Bug fix (non-breaking change which fixes an issue)
• 🧹 Chore (technical task, refactoring, maintenance)

Description

This PR revamps the email notification system to align with the application's Dark Mode branding and fixes configuration issues regarding verification links.

Key Changes:

1. Centralized Theming: Introduced src/app/core/theme.py to define the color palette (Primary Green, Dark Slate backgrounds) matching the frontend. Injected this theme object into the Jinja2 global context.
2. UI Overhaul: Refactored base.html and all child templates (content.html) to use dynamic theme variables.
3. Card Design Fix: Removed the outer dark background wrapper from base.html. Emails now render as a clean "floating card" against the email client's default background, improving aesthetics and readability.
4. HTML Cleanup: Replaced obsolete HTML attributes (e.g., cellpadding, width) with modern inline CSS to satisfy linter requirements and improve client compatibility.
5. Configuration Fixes:
   • Updated .env-dist to include http:// in BASE_SITE_URL (fixing broken redirect links).
   • Updated templates and README to use verification_token instead of the deprecated user_uuid.

Related Issue

Closes # (Insert issue number if available)

How Has This Been Tested?

• Manual Testing: Verified the HTML output of the templates to ensure the "Card" design renders correctly without the outer background box.
• Manual Testing: Confirmed that the BASE_SITE_URL now correctly redirects users when clicking action buttons.
• Unit Tests: Existing tests passed (updated context keys in tests/README).

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Screenshots (if applicable)