chore: Provide IBM Plex Sans to fix story app lag to nix (#2592)

## Summary

- add IBM Plex Sans to the Nix development environment
- provide a deterministic Fontconfig configuration when running the
story app with Cargo
- apply the same font configuration to the packaged executable wrapper

## Why

GPUI resolves the Linux system UI font to IBM Plex Sans. On NixOS, that
font was not available inside the development shell, so text rendering
repeatedly took the missing-font fallback path. With `RUST_BACKTRACE=1`
enabled by the shell, those fallback errors also captured backtraces and
caused high single-thread CPU usage during scrolling and other UI
interactions.

Providing the expected font removes that hot path. In an 8-second
runtime sample, CPU time dropped from approximately 4.05 seconds to 0.97
seconds, and profiling no longer showed unwind/backtrace work.

## Testing

- `nix flake check --no-build`
- `nix develop --command cargo run --release`
- verified `fc-match "IBM Plex Sans"` resolves to
`IBMPlexSans-Regular.otf` inside the dev shell
- `nix build .#defaultPackage.x86_64-linux`
- launched the packaged executable through its generated wrapper

Related issue: #1621