fix(frontend/pdfjs): ensure CID character rendering via CMaps & stabilize PDF compare/preview

[Ludy87]

Description of Changes

What was changed

• Introduced a shared PDFJS_DEFAULT_OPTIONS object and applied it across frontend modules using PDF.js:
  • Sets cMapUrl, cMapPacked, and standardFontDataUrl so PDF.js can correctly load CMaps and standard fonts.
  • Switches all GlobalWorkerOptions.workerSrc usages to the dynamic pdfjsPath + 'pdf.worker.mjs'.
• Exposed pdfjsPath globally in navbar.html to support deployments under subpaths/reverse proxies.
• Updated multiple pages and utilities to use the new defaults:
  • DecryptFiles.js, downloader.js, merge.js, Multi-Tool (PdfContainer.js), and feature pages (add-image.js, adjust-contrast.js, change-metadata.js, crop.js, pdf-to-csv.js, sign.js, rotate-pdf.html, convert/pdf-to-pdfa.html, merge-pdfs.html).
• Comparison tool hardening:
  • Added robust worker protocol (type: 'COMPARE' | 'SET_*') and safer logs.
  • Improved text tokenization, adaptive batch diffing with overlap de-duplication, and color fallbacks.
  • Early validation for empty/oversized/invalid PDFs with clearer user messages.
  • Disabled PDF.js worker in specific templates where legacy CMap handling caused issues (disableWorker: true) to prevent rendering failures.
  • UI/UX tweaks: processing state on the compare button, progress hints during text extraction, and more resilient error handling.
• Fixed relative path to popularity data (./files/popularity.txt) to respect base paths.

Why the change was made

• PDFs using CID fonts (e.g., CJK and other complex scripts) were rendering with missing glyphs or falling back incorrectly because CMaps and standard font data were not being provided to PDF.js. Providing proper CMap and font resources resolves CID character visibility issues and related console warnings.
• Some environments (subpath deployments, reverse proxies) broke PDF.js worker/static asset resolution; centralizing pdfjsPath and using it consistently fixes this.
• The comparison feature struggled with large/complex documents and lacked robust validation; improvements reduce timeouts, improve accuracy, and provide clearer feedback.

Closes #4391

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Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

• I have tested my changes locally. Refer to the Testing Guide for more details.

[Copilot]

Pull Request Overview

This PR refactors PDF.js usage across the application to improve reliability and maintainability. The changes introduce centralized PDF.js configuration with CMap and font options, update worker path references to use a global pdfjsPath variable, and enhance error handling in the PDF comparison feature.

• Centralized PDF.js configuration with PDFJS_DEFAULT_OPTIONS across multiple files
• Updated PDF.js worker paths from hardcoded strings to use pdfjsPath variable
• Improved error handling and validation in PDF comparison feature
• Refactored PDF comparison worker to use diff.js library instead of custom LCS algorithm

Reviewed Changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
rotate-pdf.html	Added module import for PDF.js and PDFJS_DEFAULT_OPTIONS configuration
misc/compare.html	Major refactor: added error handling, PDF validation, worker timeout, and improved structure
merge-pdfs.html	Updated worker path to use pdfjsPath variable
fragments/navbar.html	Defined global pdfjsPath constant for consistent path resolution
convert/pdf-to-pdfa.html	Updated worker path to use pdfjsPath variable
js/pages/*.js	Added PDFJS_DEFAULT_OPTIONS and updated worker paths in sign, pdf-to-csv, crop, change-metadata, adjust-contrast, add-image files
js/multitool/PdfContainer.js	Added PDFJS_DEFAULT_OPTIONS and updated PDF loading
js/merge.js	Added PDFJS_DEFAULT_OPTIONS and worker path update
js/homecard.js	Fixed fetch path from absolute to relative
js/downloader.js	Added PDFJS_DEFAULT_OPTIONS and updated worker paths
js/compare/pdfWorker.js	Refactored to use diff.js library, improved message handling, and enhanced batching logic
js/DecryptFiles.js	Added PDFJS_DEFAULT_OPTIONS to decrypt operations

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