fix(open-app): capture and display exit status when opening file in default app

When the 'open in default app' button is clicked for a binary placeholder,
we now run the OS command (open/explorer/xdg-open) via .output() to capture
its exit status and stderr, rather than fire-and-forget via .spawn().

If the command fails (e.g., no app handler found, LaunchServices error),
we now display a user-facing error in the status bar showing the reason,
instead of silently failing with only a tracing::warn! log line.

- Add AppMsg::OpenInDefaultAppFinished with path and result
- Add open_in_default_app() helper to run platform command and check exit
- Spawn background thread to run command, send result via message channel
- Update app.rs handler to display error in status bar on failure
- Tests: verify error message displays filename and error details

Fixes silent failure when opening files in default app (cross-platform).