Prevent Render warmup rate-limit failures during Xaman login

[xrpbanks]

Summary

• replace the aggressive 2-second health polling loop with exponential backoff
• respect Render Retry-After guidance and add a conservative fallback for HTTP 429 responses
• extend the free-demo warmup window to three minutes and tell users not to refresh repeatedly
• clarify that mobile Xaman may return in the phone's default browser
• add a regression test proving a login started in one browser can finish securely in another

Verified locally

• 201 backend tests passed
• frontend lint passed
• frontend production build and TypeScript validation passed
• legal/licensing, whitespace, and tracked-artifact release checks passed

This PR does not change WordPress content or deploy anything by itself.

[Copilot]

Pull request overview

This PR improves resilience of the Xaman login flow when the backend is warming up on Render, reducing the likelihood of rate-limit failures and making the mobile return-to-browser behavior clearer.

Changes:

• Replaces the fixed 2s backend warmup polling loop with exponential backoff and explicit HTTP 429 (Retry-After) handling.
• Extends the warmup window to 3 minutes and updates user-facing messaging to discourage repeated refreshes.
• Adds a regression test ensuring the identity callback can complete in a different browser session than the one that initiated login.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
frontend/lib/backendRequest.ts	Implements exponential backoff warmup probing and respects rate-limit retry guidance; updates timeout and error copy.
frontend/components/XamanLoginPanel.tsx	Updates UI messaging for longer warmup and cross-browser/mobile return behavior.
backend/tests/test_identity_endpoints.py	Adds a regression test verifying callback completion in a different “browser” (separate client cookies).

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