Chore: revert pngx pdf viewer to third party package

[shamoon]

Proposed change

No rush here.

After a couple of PRs got merged the third party version of this seems to be alive again and back up-to-date with ours and theres really no reason to maintain ours anymore. Of course the 'risk' is that project goes dormant again and we are limited by that but:

• pdfjs v4.x is a big upgrade and I have no intention of working on integrating that any time soon. Even if I did, I'd happily submit it to that package.
• The current pdfjs 3.11.174 is presumably the last 3.x they'll release, so we shouldn't be limited by that by relying on the third party version.
• The CVE fix is included.

As you can see there are really no substantive changes in this PR, which is great cause it makes me reasonably confident it wont break things (ugh, famous last words lately?).

Closes #(issue or discussion)

Type of change

• Bug fix: non-breaking change which fixes an issue.
• New feature / Enhancement: non-breaking change which adds functionality. Please read the important note above.
• Breaking change: fix or feature that would cause existing functionality to not work as expected.
• Documentation only.
• Other. Please explain: chore

Checklist:

• I have read & agree with the contributing guidelines.
• If applicable, I have included testing coverage for new code in this PR, for backend and / or front-end changes.
• If applicable, I have tested my code for new features & regressions on both mobile & desktop devices, using the latest version of major browsers.
• If applicable, I have checked that all tests pass, see documentation.
• I have run all pre-commit hooks, see documentation.
• I have made corresponding changes to the documentation as needed.
• I have checked my modifications for any breaking changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.19%. Comparing base (50a6181) to head (b0a23dc).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #6741   +/-   ##
=======================================
  Coverage   97.19%   97.19%           
=======================================
  Files         436      436           
  Lines       17977    17977           
  Branches     1527     1493   -34     
=======================================
  Hits        17473    17473           
  Misses        501      501           
  Partials        3        3           

Flag	Coverage Δ
backend	95.88% <ø> (ø)
frontend	98.63% <100.00%> (ø)

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[stumpylog]

Nice to drop the manual version