Enforce asset cap and surface upload-limit messaging

[yugalkaushik]

Issue:

Fixes #4009

Demo:

Changes:

Ensure backend signing rejects uploads once current size plus incoming file exceeds the 250MB limit, using fresh S3 totals to avoid stale user size.

• aws.controller.js: When signing uploads, we now use the latest S3 total plus the incoming file size and stop at the 250MB cap to prevent new assets from slipping through.
• users.js: Align client-side gating with the 250MB ceiling using inclusive checks so the UI mirrors server enforcement.
• uploader.js If the server returns 403 for a sign request, we trigger the upload-limit modal instead of failing silently.
• ErrorModal.jsx: Added an uploadLimit branch that reuses the existing size-limit copy and points users to assets to clear space.
• FileUploader.jsx: Hooks the uploader into the updated accept flow so limit errors still route through the standard modal experience.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines