feat: add migrate step to move mod folders between groups on preset change

Before step_fetch, scan all downloads/ subdirs and move any mod that
comparison.json now assigns to a different group. Matching uses steam_id
(via meta.cpp publishedid) first, normalized name as fallback.

Stale junctions in arma_dir are removed before the folder move so
step_link can re-create them pointing to the new location.

- New arma_modlist_tools/migrator.py: migrate_mod_groups()
- run.py: step_migrate(), --skip-migrate flag, wired into dispatch loop
- gui/app.py: step_migrate inserted as Step 3/5 between compare and fetch
- gui/locales.py: add step3/4/5 names (en + vi), renumber old 3->4, 4->5
- test_suite.py: 7 new migrator tests (158 total, 0 failed)

arma_modlist_tools/__init__.py

@@ -12,6 +12,7 @@ from .config  import load_config, Config
 from .compat  import is_windows, is_linux, get_os_label, fix_console_encoding
 from .reporter import build_missing_report, save_missing_report
 from .cleaner  import find_orphan_folders, folder_size
+from .migrator import migrate_mod_groups
 
 __all__ = [
     # parser
@@ -32,4 +33,6 @@ __all__ = [
     "build_missing_report", "save_missing_report",
     # cleaner
     "find_orphan_folders", "folder_size",
+    # migrator
+    "migrate_mod_groups",
 ]