server: resolve asset paths relative to executable

[gmontana]

Summary

Fixes #49 by resolving relative asset paths from the executable directory when they are not found from the current working directory.

This covers:

• relative GGUF model paths, including the default ds4flash.gguf
• relative MTP model paths
• bundled Metal source files such as metal/flash_attn.metal

Current-directory paths still take precedence, and the existing Metal source environment overrides are still tried first.

Validation

• git diff --check
• make ds4_test
• ./ds4_test --paths --server
• make ds4-server
• make ds4
• launched /Users/u1770777/repos/ds4/ds4-server --ctx 2048 --port 5556 from /tmp and confirmed it reached listening on http://127.0.0.1:5556

[gmontana]

I saw the note on #49 about possibly inlining the Metal sources at build time. This PR is meant as a smaller runtime lookup fix for the current layout: it keeps CWD precedence and existing Metal source env overrides, but lets launchers start ds4-server by absolute path from another working directory.

It also covers the default model path / MTP path case, which inlining Metal sources would not address. Happy to adjust the scope if you prefer a different split.