Fix /admin/social-auth production routing

[smshagor-dev]

Summary

Fix the Social Login admin page not opening at /admin/social-auth in non-Vercel production setups.

Root cause

social-auth.html is generated as a standalone webpack entry and Vercel already rewrites the clean URL correctly, but the Node production fallback and Apache .htaccess sent /admin/social-auth to the generic admin SPA (admin/index.html) before the standalone page could load.

Fix

• Route /admin/social-auth to admin/social-auth.html before the generic admin SPA fallback in the Node server.
• Add the same extensionless standalone mapping to Apache .htaccess.
• Keep the existing standalone admin pages (storage, calling, calling-push) consistent with the same routing behavior.
• No changes to admin auth, social-login API behavior, provider credentials, or client login flow.

Scope

Routing only: backend/server/server.js and frontend/.htaccess.

[vercel]

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