[codex] Improve chord export, diagnostics, and job reliability

[bassmicrobe]

What changed

• Added an in-app processing log viewer with per-job and session diagnostics, persisted failure/cancellation logs, and responsive desktop/mobile UI.
• Hardened job scheduling, cancellation, subprocess group cleanup, atomic metadata/registry writes, cross-process concurrency slots, and shutdown behavior.
• Improved chord detection/export with quarter-note and bar grids, MIDI markers, CSV export, and stronger benchmark reporting.
• Fixed upload cleanup, section validation, package-version lookup, stale diagnostic retention, and current Rust Clippy warnings.

Why

Long-running local separation jobs needed better observability and more reliable background behavior, while chord-guide quality and regression measurement needed stronger controls.

Validation

• uv run pytest -q (202 passed, 7 skipped)
• uv run ruff check app tests scripts
• uv run python -m compileall -q app
• JavaScript syntax checks for all static/js/*.js
• cargo test --manifest-path desktop/src-tauri/Cargo.toml (9 passed)
• cargo clippy --manifest-path desktop/src-tauri/Cargo.toml --all-targets -- -D warnings
• uv lock --check
• scripts/check-distribution-notices.sh
• In-app browser checks at 1440x900 and 390x844 with no post-reload console errors