Story 2422: Sign In Webpage Integration

[jlchilders11]

Issue: #2422

Summary & Context

Implements the V3 version of the sign in page. Note that this branches off of: #2482 to take advantage of the changes made to auth pages in general.

• Figma link: https://www.figma.com/design/5j0fQssrV9ipoU16P7hfKy/Website-Deliverables?node-id=1726-51323&t=vZWJaoznUsHhdFqw-4
• Link to components/page: http://localhost:8000/accounts/login/

Changes

• Changes the template for the sign in page to use the V3 version.
• Implements the social sign in links

‼️ Risks & Considerations ‼️

Please list any potential risks or areas that need extra attention during review/testing
There is no great way to test the social sign in locally, since it relies on Oauth that is not set up for development.

Screenshots

Self-review Checklist

• Link this PR to the related GitHub Project ticket

Frontend

• UI implementation matches Figma design
• Tested in light and dark mode
• Responsive / mobile verified
• Accessibility checked (keyboard navigation, etc.)
• Ensure design tokens are used for colors, spacing, typography, etc. – No hardcoded values
• Test without JavaScript (if applicable)
• No console errors or warnings

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[herzog0]

Besides the redirect bug, could we also strip out all legacy stuff?
That means:

1. Removing the V3LoginView from views.py;
2. Removing context["login_url"] = reverse_lazy("v3-login") from core/mixins.py;
3. Removing the path v3/accounts/login/ from config/v3_urls.py;

[herzog0]

When the V3Mixing dispatches, it sets _v3_active = False for POST requests, so it falls through the legacy login page. That means, if an error occurs during login, we're redirected to the legacy page.
To reproduce:

• Go to login page;
• Insert an email;
• Insert a wrong password and submit;