feat: add orphan mod cleanup tool with GUI integration and live-server tests
- Add arma_modlist_tools/cleaner.py: find_orphan_folders() detects @ModName
folders no longer referenced in comparison.json; uses _normalize_name from
fetcher for consistent three-level matching
- Add clean_orphans.py: CLI with --dry-run and --yes/-y flags; junction-safe
deletion via _is_junction() guard before shutil.rmtree
- Add Clean Orphans tab to gui/views/tools.py: scrollable checkbox list,
background scan/delete threads, pending-done-msg pattern for post-scan
status, EN/VI localization strings in gui/locales.py
- Add 23 unit tests (section 12), 6 E2E subprocess tests (section 13),
23 coverage-gap tests (section 14), 9 live-server fetcher tests (section 15)
- Fix leaked builtins.open mock in _test_read_os_release_parses_file
- Overall coverage: 84% → 93%; fetcher.py: 36% → 72%
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