Summary

• Switches the documented Google web sign-in flow to the OAuth2 redirect path (flutter_web_auth_2 + auth.html). The popup/iframe flow is no longer the default in the docs.
• Adds a shared Web callback page (auth.html) section under concepts/authentication/setup so GitHub, Google, and any future redirect-based IdP reference one canonical place.
• Rewrites the Google Web subsection, OAuth client URI table, passwords.yaml examples (dev + prod), initializeGoogleSignIn example (clientId + redirectUri under kIsWeb), and the Publishing-to-production redirect URI to use /app/auth.html (the standard template path).
• Updates the Google troubleshooting page for redirect-mode failure modes (callback never returns, clientId is required, redirect_uri_mismatch across all three configuration spots) and drops popup-only entries.
• Updates GitHub setup to reference the shared auth.html section instead of inlining the snippet.
• Adds a "Configuring the Web redirect URI" subsection to Google customizations covering the --dart-define pattern.

Supersedes #494. Most of the redirect-flow content originated from @vfiruz97's work there; this PR re-applies that intent on top of the recently merged #490.

Requires serverpod_auth_idp_flutter >= 3.5.0-beta.7 (the version that adds the redirectUri parameter to initializeGoogleSignIn).