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_api_snapshots.py

@@ -117,8 +117,20 @@ class TestSettingsHandlerSnapshots:
     """Snapshot tests for SettingsHandler responses."""
 
     @pytest.mark.asyncio
-    async def test_get_settings_response_format(self, snapshot: SnapshotAssertion):
+    async def test_get_settings_response_format(
+        self, snapshot: SnapshotAssertion, monkeypatch: pytest.MonkeyPatch
+    ):
         """Verify get_settings response format matches snapshot."""
+        # Pin the resolved sidecar root: it derives from the machine-specific
+        # settings directory, which would make the snapshot non-deterministic.
+        monkeypatch.setattr(
+            "py.routes.handlers.misc_handlers.describe_sidecar_root",
+            lambda: {
+                "root": "/sidecars",
+                "is_default": True,
+                "inside_repo": False,
+            },
+        )
         settings_service = DummySettings({
             "civitai_api_key": "test-key",
             "language": "en",