# Fix Failing Mock-Network UI Tests

## Problem
2 of 11 new UI tests fail — both are Phase 3 mock-network tests:
1. `testAlbumTracksWithMockData` — fails at line 198: "Album category should be visible with mock data"
2. `testDecodingErrorShowsMessage` — fails at line 186: "Should show either Not Connected or Browse section"

Both fail because the `-MockNetwork` launch argument isn't being picked up by the app to register the `MockURLProtocol`, so the mock data never reaches the views.

## Goal
Both mock-network tests pass when run via `xcodebuild test`.

## Non-goals
- No changes to Phase 1/2 tests (they pass)
- No changes to the existing 20 UI tests (they pass)
- No new test scenarios

## Acceptance Criteria
- [ ] `testAlbumTracksWithMockData` passes
- [ ] `testDecodingErrorShowsMessage` passes
- [ ] All other tests still pass (311 unit + existing UI + Phase 1/2)
- [ ] `-MockNetwork` launch arg correctly registers MockURLProtocol in the app

## Appetite
- 2-3 files changed max
- Fix the wiring, don't redesign the mock approach

## Investigation needed
1. Check `MixBoardApp.swift` — does it check for `-MockNetwork` launch argument?
2. Check `MockURLProtocol` — is it in the app target (Sources/) or only in the test target (UITests/)?
3. Check the test files — are they passing `-MockNetwork` via `app.launchArguments`?
4. The issue is likely: MockURLProtocol exists only in UITests target, so the app can't see it. Fix: either move mock registration to the app target with conditional compilation, or use a different approach.

## Technical Constraint
MockURLProtocol must be available to the main app target (not just UITests) for runtime URL interception to work. Use `#if DEBUG` guard so it's stripped from release builds.