feat(sidecars): surface storage location and cover excluded models in migration

After migrating to centralized sidecar storage users had no indication
where their files went, and portable-mode installs silently placed the
sidecar root inside the plugin folder where a reinstall or git clean
would delete it.

- Migration now also covers models excluded from the library view and
  returns the resolved sidecar root in its result payload
- get_settings exposes the resolved sidecar root, whether it is the
  default, and whether it lives inside the installation folder
- New POST /api/lm/sidecars/open-location endpoint opens (or copies)
  the sidecar storage folder
- Settings UI always shows the effective storage path with an
  open-folder button, and warns when the root is inside the
  installation folder (portable-mode hazard)
- Migration confirmation shows the destination; on completion a result
  dialog summarizes moved/skipped/conflict counts with the storage
  location and an open-folder action
- Ignore /sidecars/ at the repository root so portable-mode sidecars
  are never committed

Refs #1045

tests/routes/__snapshots__/test_api_snapshots.ambr

@@ -31,6 +31,9 @@
       'language': 'en',
       'llm_api_key_set': False,
       'other_models_paths_available': False,
+      'sidecar_storage_root': '/sidecars',
+      'sidecar_storage_root_in_repo': False,
+      'sidecar_storage_root_is_default': True,
       'standalone_mode': False,
       'theme': 'dark',
     }),