Support for Alternative Package Managers (mise, asdf) and Custom Bun Paths

Description

Currently, Script Kit discovery for the Bun executable relies on a hardcoded list of common global paths (Homebrew, /usr/local/bin, etc.) and the system PATH in file src/executor/runner.rs.

While this works for standard installations, it fails for users managing their environments via tools like mise/asdf or managers not hardcoded, and even if the user defined the path in the config.ts file it was not being respected because the path was not one of the hardcoded.

Problem

• The "Bun not installed" warning banner persists even if Bun is managed by mise/asdf, unless the user manually launches the app from a terminal with a fully initialized environment.
• Discovery logic was fragmented across preflight.rs, app_run_setup.rs, and runtime_watchers_scheduler.rs, making it difficult to keep path resolution consistent.

Start by tracing Bun executable discovery in src/executor/runner.rs and compare the related logic in preflight.rs, app_run_setup.rs, and runtime_watchers_scheduler.rs. Check how config.ts paths and initialized environments are handled; done means discovery is consistent across these locations and the warning no longer persists for supported custom or managed Bun paths.