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/frontend/i18n/src/resources/en.json

@@ -2177,6 +2177,7 @@
   "error.SSRF_BLOCKED_ERROR": "Invalid URL",
   "error.RESPONSE_TOO_LARGE_ERROR": "Response too large ({{receivedBytes}} bytes), limit is {{limitBytes}} bytes",
   "error.EMAIL_SERVICE_NOT_CONFIGURED": "Email service is not configured.",
+  "error.IMAGE_FORMAT_NOT_SUPPORTED": "Image format not supported: {{format}}",
   "error.QUERY_TOO_LONG": "Query is too long, max length is {{max}}.",
   "error.VALIDATION_ERROR": "Validation error, errors: {{errors}}",
   "error.USER_NOT_FOUND": "User not found.",