Improve catalogue and PDF viewer UX and light mode uiux

[Rithish-2914]

📌 Purpose

Improve the overall user experience across the PDF viewer and paper catalogue, with a focus on Reading Mode, light-mode UI consistency, bulk downloads, and clearer loading/feedback states.

Showcase

• PDF Reading Mode now works as a viewport-level overlay.
• Improved light-mode UI and dropdown readability.
• Improved Select All / Deselect All and Download Selected interactions.
• Added clear loading feedback while preparing downloads.

Changes

PDF Viewer / Reading Mode

• Added/improved Reading Mode with a dedicated book icon.
• Reading Mode now covers the browser viewport using a fixed overlay.
• The banner/navbar underneath is automatically covered while Reading Mode is active.
• Added body scroll locking while Reading Mode is active.
• Reading Mode respects the browser viewport, including split-screen/snapped windows.
• Reading Mode does not use the browser's native Fullscreen API.
• Added visual feedback/coachmark for the new Reading Mode feature.
• Added keyboard shortcut support for Reading Mode.

Light Mode UI/UX

• Fixed white text appearing on light dropdown backgrounds.
• Improved dropdown readability in light mode.
• Updated hover/focus states for better contrast.
• Improved consistency between light and dark mode controls.

Catalogue Selection & Downloads

• Improved Select All / Deselect All behaviour.
• Added selected-paper count to the Download button.
• Improved Download Selected button styling and interaction.
• Download button is disabled when no papers are selected.
• Added loading state while the ZIP is being prepared.
• Added spinner and Preparing zip... feedback during downloads.
• Prevented duplicate download actions while a download is in progress.
• Improved download error/partial-download feedback.
• Improved overall bulk-selection and download UX.

Additional Notes

• Changes were tested locally before pushing.
• Reading Mode is implemented as a CSS viewport overlay rather than native browser fullscreen.
• PR targets the staging branch.