Enhance authentication flow with GitHub and Cloudflare support

[AnkanSaha]

This pull request updates the authentication flow to support both Google and GitHub as sign-in providers, consolidates the sign-in and sign-up panels into a single component, and improves user feedback for authentication events. It also updates various pages and tests to reflect these changes and improves error handling and UI consistency.

Authentication provider enhancements:

• The GoogleAuthPanel component now supports both Google and GitHub sign-in, with appropriate UI and busy state handling for each provider. The sign-in and sign-up flows are unified into a single panel with updated copy. (client/src/components/auth/GoogleAuthPanel.tsx [1] [2] [3] [4] [5]
• Added a GithubMark icon for GitHub authentication to AuthLayout. (client/src/components/auth/AuthLayout.tsx client/src/components/auth/AuthLayout.tsxR129-R134)

Testing updates:

• Tests for GoogleAuthPanel are updated and expanded to cover GitHub sign-in, busy state handling, and unified sign-in logic. Mocks for GitHub provider are added. (client/src/__tests__/components/auth/GoogleAuthPanel.test.tsx [1] [2] [3]; client/src/__tests__/components/auth/PasskeyLogin.test.tsx [4] [5]; client/src/__tests__/contexts/AuthContext.test.tsx [6] [7]
• Added a test to ensure an error is shown if the sign-in popup is closed without completing. (client/src/__tests__/contexts/AuthContext.test.tsx client/src/tests/contexts/AuthContext.test.tsxR98-R129)

Page and navigation changes:

• The /register page now redirects to /login, and the /login page uses the unified GoogleAuthPanel. (client/src/app/register/page.tsx [1] client/src/app/login/page.tsx [2]
• Updated call-to-action links on the blog and testimonials pages to point to /login instead of /register. (client/src/app/blog/[slug]/page.tsx client/src/app/blog/[slug]/page.tsxL128-R128, client/src/app/testimonials/page.tsx client/src/app/testimonials/page.tsxL34-R34)

Error and notification handling:

• Added toast notifications for Cloudflare OAuth connection and error states on the onboarding and load balancer pages. (client/src/app/onboarding/page.tsx [1] [2]; client/src/app/loadbalancers/page.tsx [3]

These changes streamline authentication, improve the user experience, and ensure tests cover the new logic.