Allow local and dev origins in MCP Apps sandbox referrer checks

[fengfeng-zi]

Summary

• replace the hardcoded localhost-only referrer regex with helper-based validation for local and dev hosts
• allow loopback, .localhost, .local, and RFC1918 private IPv4 origins while keeping existing http/https and parent-origin checks
• tighten the rejection message to reflect the intended local/dev-only policy

Closes #1276

Testing

• Not run locally in this environment