Make MAX_COLLABORATORS configurable via environment variable

[coderanik]

Description

The collaborator limit in the share modal is hardcoded as MAX_COLLABORATORS = 3. This should be configurable via an environment variable so self-hosters can change it without modifying code.

Location

frontend/src/components/ShareModal.tsx:7

Suggested Fix

1. Move the constant to a config file (e.g., frontend/src/lib/constants.ts)
2. Read from process.env.NEXT_PUBLIC_MAX_COLLABORATORS with a fallback of 3
3. Optionally fetch the actual limit from a backend endpoint

Difficulty: Easy

Estimated Time: 30 minutes