Enables running multiple CLIProxy variants simultaneously on different
ports. Each variant now gets a unique port in the 18100-18199 range,
allowing concurrent use of providers like Gemini + Codex + custom variants.
Key changes:
- Add port field to variant config schema
- Implement automatic port allocation (18100 + variant index)
- Support variant-specific settings paths in config generator
- Display port in dashboard UI for debugging
- Show all models in variant editor dropdown
- Add comprehensive tests for port allocation edge cases
Closes related variant isolation work.
