feat: add multi-select filters with URL synchronization and update tests

[Utkarsha155]

Pull Request description

This PR introduces multi-select filtering for region and category, enhancing the flexibility of event discovery.

Previously, users could only select a single region or category at a time. This update allows selecting multiple options simultaneously while maintaining URL synchronization, enabling persistence across refresh and shareable filtered links.

Solves issue: #103

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How to test these changes

• Run:

  npm install
  npm run dev
  

• Open browser at:

  http://localhost:5173/du-event-board/
  

• Try selecting multiple regions and categories from dropdown

• Verify:

  • Multiple selections are applied correctly
  • URL updates (e.g., ?region=India,US)
  • Refresh retains selected filters

• Run tests:

  npm test
  

• Ensure all tests pass successfully

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Pull Request checklists

This PR is a:

• bug-fix
• new feature
• maintenance

About this PR:

• it includes tests.
• the tests are executed on CI.
• pre-commit hooks were executed locally.
• this PR requires a project documentation update.

Author's checklist:

• I have reviewed the changes and it contains no misspelling.
• New and old tests passed locally.

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Additional information

Key Changes:

• Added multi-select dropdown UI using checkboxes
• Extended useUrlState to support array values (comma-separated)
• Updated filtering logic to handle multiple selections
• Updated test cases to align with new UI behavior

Benefits:

• Improved user experience with flexible filtering
• Persistent filters via URL
• Shareable filtered views

Screenshots:

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Reviewer's checklist

## Reviewer's Checklist

- [ ] I managed to reproduce the problem locally from the `main` branch
- [ ] I managed to test the new changes locally
- [ ] I confirm that the issues mentioned were fixed/resolved.

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[yuvimittal]

@Utkarsha155 , i am not really sure if this feature is important, also could you rebase your branch!?

[Utkarsha155]

@Utkarsha155 , i am not really sure if this feature is important, also could you rebase your branch!?

Hi @yuvimittal ,

Thanks for the review.

I have rebased the branch with the latest main and resolved all issues. All checks are now passing.

Please let me know if you’d like any changes or refinements.