feat: add WebDAV sync and advanced editor

[XianYuDaXian]

Summary

• add WebDAV-based remote sync with local/remote storage mode switching
• add conflict-aware sync helpers, overwrite actions, cleanup/init actions, and remote change retry flow
• add an advanced reference image editor for text, brush fill, region fill, image layers, flip controls, and mobile interactions
• integrate the advanced editor into the upstream mask editor flow

Notes

• the upstream mask editor remains responsible for edit-mask generation for OpenAI image edit requests
• the advanced editor is used for local visual edits that are saved back into reference images before submission

Test

• npm run build
• npm test