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/test_misc_routes.py

@@ -533,6 +533,58 @@ async def test_open_backup_location_uses_settings_directory(tmp_path, monkeypatc
     assert calls == [["xdg-open", str(backup_dir)]]
 
 
+@pytest.mark.asyncio
+async def test_open_sidecar_location_opens_configured_root(tmp_path, monkeypatch):
+    from py.services.settings_manager import get_settings_manager
+
+    root = tmp_path / "sidecars"
+    get_settings_manager().set("sidecar_storage_path", str(root))
+
+    handler = FileSystemHandler(settings_service=SimpleNamespace())
+
+    calls = []
+
+    def fake_popen(args):
+        calls.append(args)
+        return MagicMock()
+
+    monkeypatch.setattr(subprocess, "Popen", fake_popen)
+    monkeypatch.setattr("py.routes.handlers.misc_handlers._is_docker", lambda: False)
+    monkeypatch.setattr("py.routes.handlers.misc_handlers._is_wsl", lambda: False)
+
+    response = await handler.open_sidecar_location(FakeRequest())  # pyright: ignore[reportArgumentType]
+    payload = _json_payload(response)
+
+    assert response.status == 200
+    assert payload["success"] is True
+    assert payload["path"] == str(root)
+    # Created on demand so the button works before any migration ran.
+    assert root.is_dir()
+    assert calls == [["xdg-open", str(root)]]
+
+
+@pytest.mark.asyncio
+async def test_get_settings_includes_resolved_sidecar_root(tmp_path):
+    from py.services.settings_manager import get_settings_manager
+
+    root = tmp_path / "sidecars-custom"
+    get_settings_manager().set("sidecar_storage_path", str(root))
+
+    handler = SettingsHandler(
+        settings_service=DummySettings(),
+        metadata_provider_updater=noop_async,
+        downloader_factory=dummy_downloader_factory,
+    )
+
+    response = await handler.get_settings(FakeRequest())  # pyright: ignore[reportArgumentType]
+    payload = _json_payload(response)
+
+    assert payload["success"] is True
+    assert payload["settings"]["sidecar_storage_root"] == str(root)
+    assert payload["settings"]["sidecar_storage_root_is_default"] is False
+    assert payload["settings"]["sidecar_storage_root_in_repo"] is False
+
+
 @pytest.mark.asyncio
 async def test_open_settings_location_headless_returns_clipboard_mode(tmp_path, monkeypatch):
     """Without a GUI session xdg-open cannot work; the handler must hand the