upstream PR #15684: project file failures show untranslated server reasons #19

Follow-up from LibreChat-AI#15684. Originally filed as LibreChat-AI#15805 (to be closed there; follow-ups live on this fork).

What happens

When attaching a file to a Chat Project fails, the failed row renders the server's reason verbatim:

// client/src/components/Projects/ProjectResources.tsx
const associationError = responseData && 'error' in responseData ? responseData.error : undefined;
return typeof associationError === 'string' && associationError.trim().length > 0
  ? associationError
  : localize('com_error_files_upload');

Showing the real reason is the point — a capacity race, an expired file or a deleted project each explain a retry that will not succeed, and the generic upload error does not. But those strings come from packages/api/src/projects/handlers.ts in English (Project file limit reached, Project not found), so a user on a non-English locale gets one English line in an otherwise localized surface. Only the fallback is localized.

Suggested direction

Give the project file endpoints a small stable reason code alongside the message and map codes to localized strings on the client, keeping the server string as the fallback for unknown codes. That keeps the diagnostic value without shipping untranslated UI text.

Found while auditing PR LibreChat-AI#15684, which introduced the structured reason path.