Url attachments (#1965)
* Add custom scrollbar class to Modal component
Applied the 'scrollbar-custom' class to the Modal's main container to enable custom scrollbar styling. This improves the appearance and consistency of scrollbars within modals.
* Add support for loading attachments from URLs
Implements the ability to load file attachments via URL parameters using a new utility (loadAttachmentsFromUrls) and a server-side proxy endpoint (/api/fetch-url) to securely fetch remote files. Updates chat and model pages to handle 'attachments' query parameters, injects file content into user messages, and improves multimodal message preparation. Also includes minor UI adjustments for file display and refactors Plausible analytics script initialization.
* Block redirects in fetch-url API endpoint
