Validate proxy and gateway port settings

[rissrice2105-agent]

Closes #22

Summary

• require complete decimal port strings instead of accepting parseInt prefixes
• reject zero, negative, fractional, non-numeric, and values above 65535
• preserve whitespace trimming and documented defaults
• cover malformed inputs and both valid range boundaries

Validation

• node --test tests/config.test.ts (2/2)
• focused TypeScript check for lib/config.ts and tests/config.test.ts
• git diff --check