feat(server): add image resize support (#14588)

fix #13842 


#### PR Dependency Tree


* **PR #14588** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Images are now processed natively and converted to WebP for smaller,
optimized files; Copilot and avatar attachments use the processed WebP
output.
* Avatar uploads accept BMP, GIF, JPEG, PNG, WebP (5MB max) and are
downscaled to a standard edge.

* **Error Messages / i18n**
  * Added localized error "Image format not supported: {format}".

* **Tests**
* Added end-to-end and unit tests for conversion, EXIF preservation, and
upload limits.

* **Chores**
  * Added native image-processing dependencies.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/native/src/lib.rs

@@ -7,6 +7,7 @@ pub mod doc_loader;
 pub mod file_type;
 pub mod hashcash;
 pub mod html_sanitize;
+pub mod image;
 pub mod llm;
 pub mod tiktoken;