Document FILE_IMAGE_COMPRESSION_WIDTH and FILE_IMAGE_COMPRESSION_HEIGHT

[ThomasCle]

Summary

Add missing documentation for the following environment variables in docs/reference/env-configuration.mdx:

• FILE_IMAGE_COMPRESSION_WIDTH
• FILE_IMAGE_COMPRESSION_HEIGHT

What this changes

• Documents both variables as ConfigVar environment variables
• Adds their default value (None)
• Clarifies that they control client-side compression limits for chat-uploaded image files
• Notes that non-image file uploads are not affected

Why

These variables exist in Open WebUI but were not documented in the environment variable reference. This PR fills that gap so users can understand what they do and when they apply.

Notes

This wording was checked against current upstream behavior:

• the backend exposes these values through app config
• the frontend uses them during chat image upload compression
• aspect ratio is preserved during resizing