Fix: add stable redirect_uri for Google OAuth to avoid redirect_uri_mismatch

[ashfaq3112]

Hello.js currently generates a dynamic redirect URI based on window.location,
which causes Google OAuth to return redirect_uri_mismatch unless the user manually
copies the redirect URL into Google Cloud Console for each page.

This PR introduces a stable redirect URI for the Google provider:

redirect_uri: window.location.origin + '/redirect.html'

This makes Google OAuth predictable, prevents mismatch errors, and follows how
modern libraries like Firebase and Auth0 use a fixed callback path.

Changes:

• Added stable redirect_uri to Google module
• Minor whitespace change in redirect.html
• Added test demo file for clarity during development

Fixes #