feat: add PDF navigation history controls

[FP2003]

Summary

Adds Back and Forward buttons to the PDF toolbar for navigating between internal link sources and destinations.
This was suggested in #43 (Issues section)

For extra context, it seems like you were already able to press on the contents and navigate but there was no controls/history to be able to return.

Changes

File	Changes
assets/main.mjs	Creates the navigation controls, connects them to PDF.js history, and updates their enabled state using viewer events.
assets/main.css	Adds icon-mask styling for the new toolbar buttons.
assets/navigation-back.svg	Adds the Back toolbar icon.
assets/navigation-forward.svg	Adds the Forward toolbar icon.
assets/pdf.js/web/viewer.mjs	Enables PDF.js history in the embedded VS Code webview, exposes Back/Forward availability, and clears Forward history after branching.
patches/pdf.js.patch	Records the PDF.js modifications so they are preserved during future PDF.js updates.

Behaviour

• Buttons are initially disabled when no navigation history exists.
• Following an internal PDF link enables Back navigation.
• Back returns to the previous source position.
• Forward returns to the linked destination.
• Multiple internal jumps can be traversed in both directions.
• Following a new link after navigating Back clears the previous Forward path.
• Exact PDF destinations, including position and zoom, are preserved by PDF.js history.
• The controls use PDF.js’s existing toolbar styling and responsive behaviour.

Validation

• Manually tested in a VS Code Extension Development Host with internal PDF academic files content pages.
• pnpm run lint passes.
• pnpm run build passes.
• PDF.js patch dry-run succeeds.
• JavaScript syntax and Git whitespace checks pass.