fix: address review — injective mirror roots, root relocation, full preview coverage, EXDEV-safe rollback

Codex review on #1124:

- P1: mirror layout root component is now <basename>-<roothash>
  (sha256 of the normalized root path), so two roots sharing a basename
  no longer map to the same mirror directory and overwrite each other's
  sidecars
- P1: changing sidecar_storage_path while centralized no longer strands
  assets in the old root — new relocate_root migration direction moves
  the whole mirror tree, rewrites preview_url prefixes inside sidecars,
  reconciles scanner caches, and prunes the emptied old tree; the
  settings UI detects the path change and offers the relocation
- P2: migration enumerates the same preview candidates as
  find_preview_file — case-insensitive variants (model.WEBP) and the
  legacy .example.0.jpeg suffix — instead of exact lowercase
  PREVIEW_EXTENSIONS only
- P2: _rollback_model_staging restores staged files with the
  EXDEV-tolerant mover, so a failed undoable-delete staging no longer
  strands a cross-filesystem centralized sidecar copy

Tests: same-basename root injectivity, mixed-case/example preview
migration, relocate_root happy path + guards + route 400, frontend
relocation prompt flow. Verified end-to-end in a sandboxed standalone
server: uppercase/legacy previews migrate, root relocation moves the
tree and the list API serves the new locations immediately without a
rescan.

templates/components/modals/settings/library.html

@@ -354,7 +354,7 @@
                     <div class="text-input-wrapper">
                         <input type="text" id="sidecarStoragePath"
                                placeholder="{{ t('settings.sidecarStorage.pathPlaceholder') }}"
-                               onblur="settingsManager.saveInputSetting('sidecarStoragePath', 'sidecar_storage_path')"
+                               onblur="settingsManager.handleSidecarStoragePathChange()"
                                onkeydown="if(event.key === 'Enter') { this.blur(); }" />
                     </div>
                 </div>