Ramsundar - fix: prevent duplicate pr number entries on pr grading screen

[Ram-blip]

Description

Related PRS (if any):

This frontend PR is related to the development branch backend.

Main changes explained:

• Before adding a PR, the screen now checks the current reviewer’s list (with normalized comparison) and shows an error and blocks the add if that PR number is already there; the error is shown in the UI and works in dark mode.

How to test:

1. check into current branch
2. do npm install and ... to run this PR locally
3. Clear site data/cache
4. log as admin user
5. Go to /pr-grading-screen
6. Click "Add new" button for each user and check with existing Pr's number for the particular user, an error line appears if you type an existing pr.

Screenshots or videos of changes:

Note:

Include the information the reviewers need to know.

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[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud