DID changes in pdfviewer for new zen mode

[Rithish-2914]

📌 Purpose

This PR introduces Zen Mode in the PDF viewer to provide a cleaner, distraction-free reading experience.

The goal is to allow users to focus only on the paper content without needing to enter browser fullscreen or interact with unnecessary surrounding UI.

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Corresponding issue: closes #522

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🖼️ Showcase

Before

The PDF viewer displays the regular navigation and surrounding UI.

Zen Mode

Zen Mode provides a focused reading experience where the paper remains the primary visible content while unnecessary UI is minimized.

Add screenshots/screen recording here.

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🔧 Changes

• Added a Zen Mode toggle to the PDF viewer
• Added support for entering/exiting Zen Mode independently of browser fullscreen
• Kept existing PDF navigation, scrolling, zoom, and page controls working
• Integrated Zen Mode with the existing fullscreen state
• Updated the PDF viewer to work with the latest staging changes

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➕ Additional Notes

• Zen Mode does not replace the existing fullscreen functionality.
• The existing PDF viewer behavior is preserved outside Zen Mode.
• Tested PDF navigation, scrolling, zoom, fullscreen, and Zen Mode interactions.