Add troubleshooting guide for cache-clear recovery

[Miracle656]

Summary

When users clear their browser cache, they lose the fee-payer secret. The wallet has graceful recovery, but the user needs to know what's recoverable and what isn't. Add a troubleshooting page.

Files

• new docs/troubleshooting.md
• link from frontend/wallet/README.md

Acceptance criteria

• Explains: contract XLM (C...) is safe, fee-payer XLM (G...) is lost
• Step-by-step: how to use the 'Set up fee-payer' button
• Notes the 5-minute lock timer and how to re-auth