feat: Verify first-run BlackHole installer flow

## What
Verify and complete the first-run BlackHole installer experience end to end.

## Why
- `.planning/codebase/CONCERNS.md` keeps bundled BlackHole setup open until the actual `.pkg` asset and first-launch flow are verified.
- The app exposes status and install actions, but system-audio setup is not complete until users can reliably install or route BlackHole.
- A broken installer path leaves system audio capture unavailable for new users.

## How
- Verify the bundled BlackHole package asset is real, versioned, and included in Tauri resources.
- Exercise first-launch setup on a clean macOS profile.
- Confirm permissions/admin prompts and post-install detection behavior.
- Update setup UI and docs with any required manual recovery steps.