feat(storage): add Apple iCloud Drive storage provider

Add iCloud Drive as a new storage destination using the pyicloud library.
Includes upload task, configuration, user integration handler, provider
status, onboarding support, and comprehensive tests.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/files.html

@@ -525,6 +525,7 @@
           <option value="webdav" {% if storage_provider == "webdav" %}selected{% endif %}>WebDAV</option>
           <option value="ftp" {% if storage_provider == "ftp" %}selected{% endif %}>FTP</option>
           <option value="sftp" {% if storage_provider == "sftp" %}selected{% endif %}>SFTP</option>
+          <option value="icloud" {% if storage_provider == "icloud" %}selected{% endif %}>iCloud Drive</option>
         </select>
       </div>