fix(sidecars): report over-long migration paths and fix root relocation (#1142)

- Surface Windows WinError 206 / ENAMETOOLONG during sidecar migration as
  an actionable per-model error instead of a bare OSError
- Offer root relocation based on the resolved sidecar root, so moving off
  (or back to) the default storage directory relocates existing sidecars
- Hint in the migration confirm dialog when the destination is the default
  location, so a custom folder can be set before migrating

templates/components/modals/confirm_modals.html

@@ -104,6 +104,7 @@
         <h2 data-role="title"></h2>
         <p class="delete-message" data-role="message"></p>
         <p class="delete-message sidecar-migration-destination" data-role="destination" style="display: none;"></p>
+        <p class="delete-message sidecar-migration-hint" data-role="default-location-hint" style="display: none;"></p>
         <div class="modal-actions">
             <button class="cancel-btn" data-action="cancel-sidecar-migration">{{ t('common.actions.cancel') }}</button>
             <button class="primary-btn" data-action="confirm-sidecar-migration"></button>