Add local backend connection docs to CLAUDE.md

[cttsai-stripe]

Summary

• Documents how to connect the paymentsheet-example playground to a local backend server
• Adds gradle.properties flags for backend URL and cleartext traffic

Test plan

• Verify local backend connection works with documented settings

🤖 Generated with Claude Code

[jaynewstrom-stripe]

Worth linking the trailhead doc?

[cttsai-stripe]

Worth linking the trailhead doc?

Do we have a trailhead for this?
I only saw the slack message that includes the PR. I could create one though.

[jaynewstrom-stripe]

I was thinking https://trailhead.corp.stripe.com/docs/mobile-sdk/testing-paymentsheet-with-devbox#get-the-playground-running-on-local-machine which hasn't been completely updated yet. But might be worth throwing the new info in and linking it.