Attach custom headers to Turbo fetch requests (stimulus-free)

Sometimes you need to attach a header to every Turbo request (like a feature-flag variant, or a client version). Turbo emits turbo:before-fetch-request, which lets you mutate the outgoing request before it is sent. I keep the handler tiny and global (in application.js) rather than wiring Stimulus everywhere. This is also a clean place to attach debugging headers in development. The key is to avoid anything that breaks caching or security: don’t put secrets in headers, and be mindful that headers can affect CDNs. If you need per-request headers, prefer a Stimulus controller, but for app-wide metadata this event is perfect.