fix(desktop): explain missing local catalog

[heomin86]

Summary

• replace the misleading transcription-approval phase with a local-catalog requirement
• report the exact catalog search path and required artifact layout when no source SHA-256 matches
• state explicitly that external files are not auto-imported and that no agents, video mutation, or provider spend occurred
• map the new response fields through the desktop service and cover the contract with a regression test

Verification

• uv run pytest tests/test_desktop_runtime.py::test_unprepared_source_requires_transcription_approval -q — 1 passed
• npm run build in desktop/ — passed
• python3 -m py_compile reelbrain/fanout.py tests/test_desktop_runtime.py — passed
• git diff --check — passed

Existing unrelated test failures

• the full Python suite currently reports 282 passed, 1 skipped, and 1 existing failure in test_renders_centered_blurred_bilingual_short_with_exact_text because RuntimeGuard calls Path.exists() on a long FFmpeg filter string (OSError: File name too long)
• the Rust suite currently reports 12 passed, 2 ignored, and 2 existing fixture-dependent failures in discovers_creator_review_outputs and preflight_is_local_and_detects_audio_video

This PR does not add catalog import or transcription execution; it only makes the existing fail-closed boundary actionable and accurate.