feat: persistent pdf viewer zoom level

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Description

This change ensures that zoom levels in the pdf-viewer component persist across different PDFs and user submissions, instead of resetting to the default zoom every time.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Login as atutor or aconvenor and select the inbox/explorer tab to view a list of student submissions.
Select one of the submissions and modify the zoom level.
Select a different submission and/or refresh the page to verify that the zoom level remains the same.

Testing Checklist:

• Tested in latest Chrome
• Tested in latest Safari
• Tested in latest Firefox

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have requested a review from @macite and @jakerenzella on the Pull Request

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merged in upstream