Fix sidecar not found in bundled AppImage.

Tauri maps `..` to `_up_` when bundling resources, so `../sidecar/**/*`
ends up at `_up_/sidecar/` in the resource directory. The resolve logic
now checks both `sidecar/` and `_up_/sidecar/` prefixes before falling
back to the compile-time manifest path.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nedtwigg