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

templates/components/modals/settings/library.html

@@ -359,6 +359,23 @@
                     </div>
                 </div>
             </div>
+            <div class="setting-row">
+                <div class="setting-info">
+                    <div class="input-help sidecar-storage-location">
+                        {{ t('settings.sidecarStorage.effectivePathLabel') }}
+                        <code id="sidecarStorageResolvedPath" class="backup-location-path"></code>
+                    </div>
+                    <div class="input-help sidecar-storage-repo-warning" id="sidecarStorageRepoWarning" style="display: none;">
+                        <i class="fas fa-exclamation-triangle"></i>
+                        {{ t('settings.sidecarStorage.repoWarning') }}
+                    </div>
+                </div>
+                <div class="setting-control">
+                    <button type="button" class="secondary-btn" id="sidecarStorageOpenBtn" onclick="settingsManager.openSidecarStorageLocation()">
+                        {{ t('settings.sidecarStorage.openFolderButton') }}
+                    </button>
+                </div>
+            </div>
         </div>
 
         <div class="setting-item">