feat: optimize RPC resilience, session registration latency, and tipping UX

[JaDi03]

Summary of Changes

• RPC Configuration & Resilience:

  • Configured ARC_RPC_URL to default to official public Arc Testnet RPC (https://rpc.testnet.arc.network).
  • Allowed dynamic override via .env without modifying source code.

• Session Registration Latency:

  • Tuned indexer propagation wait loop in /register-session to 12 attempts (18s max) to prevent Nginx 502 Bad Gateway timeouts.
  • Added a 2-second client-side indexing propagation buffer before calling /register-session.

• Tipping UX Improvements:

  • Removed redundant "Just Leave" button from the tipping widget card.
  • Handled settlement failed errors in /api/core/tip as HTTP 402 (Insufficient Balance) to automatically open the deposit modal.
  • Added Close Button ("✕") and backdrop click-dismissal to the tipping onboarding modal.
  • Removed full-screen backdrop blur (backdrop-filter: none !important) and media locking (!isTipMode) in tipping mode so free videos continue playing without interruption.