UN-1720 [FIX] Refactor to resolve SonarQube duplicate return issue

[athul-rs]

Summary

• Fixed SonarQube code smell in FileUpload.jsx where beforeUpload function had multiple identical return false statements
• Consolidated logic to use a single return statement with proper conditional handling
• Added explanatory comment for why the function always returns false (to prevent automatic upload in Ant Design)

Changes

• Refactored beforeUpload function in /frontend/src/components/agency/file-upload/FileUpload.jsx
• Used if-else structure instead of multiple returns
• No functional changes - behavior remains the same

Test Plan

• [ ] File upload still works as expected
• [ ] Files are added to list when under the limit
• [ ] Error message appears when max files exceeded
• [ ] Manual upload on submit still functions correctly

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