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

py/routes/misc_route_registrar.py

@@ -120,6 +120,9 @@ MISC_ROUTE_DEFINITIONS: tuple[RouteDefinition, ...] = (
     RouteDefinition(
         "GET", "/api/lm/sidecars/migrate", "migrate_sidecars"
     ),
+    RouteDefinition(
+        "POST", "/api/lm/sidecars/open-location", "open_sidecar_location"
+    ),
     RouteDefinition(
         "POST", "/api/lm/download-model-source", "download_model_source"
     ),