fix(viewer): release sessions on early close, fail fast on dead SW, sync MIME map

[erseco]

Stack 2/5, based on #128.

What

• Viewer closed while loading: if the viewer unmounts while registerSession is in flight, teardown() runs without the session and the SW keeps the whole package in memory. The viewer now unregisters it once the await returns (this.$.isUnmounted).
• Service Worker install failure: waitForActive never settled when the worker went redundant. The viewer hung on "Preparing viewer…" and never reached the server fallback. It now rejects.
• MIME map: the PHP fallback map lacked xhtml xml txt ico m4a m4v oga ogv otf pdf, so in fallback mode those were served as application/octet-stream + nosniff (PDFs, ogv video and otf fonts broke). A PHPUnit parity test now reads src/elpx/asset-map.ts and fails on drift.

New tests/js/service-worker-client.test.ts covers activation and the redundant rejection.

Verification

composer install                 exit 0
npm install                      exit 0
npm run typecheck                exit 0
npm test                         Tests 123 passed (123)
npm run lint                     exit 0
npm run build                    ✓ built
make architecture-check          Architecture records OK — 3 records, 0 changes.
make -n download-editor … typecheck   exit 0
vendor/bin/phpunit               OK (116 tests, 328 assertions)
composer cs:check                Found 0 of 32 files that can be fixed
git diff --check                 clean

[github-actions]

Preview this PR in the Nextcloud Playground

A fresh Nextcloud boots in your browser with this branch's exelearning app installed and enabled (log in as admin / admin). Two sample .elpx are seeded under exelearning-samples/ in Files — click one to open the viewer.

eXeLearning editor: v4.0.5 (overlaid at boot from the upstream release).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.79%. Comparing base (b04449b) to head (cb54da0).

Additional details and impacted files

@@                      Coverage Diff                       @@
##             fix/asset-and-zip-hardening     #125   +/-   ##
==============================================================
  Coverage                          93.79%   93.79%           
  Complexity                           152      152           
==============================================================
  Files                                 22       22           
  Lines                                661      661           
  Branches                              54       54           
==============================================================
  Hits                                 620      620           
  Misses                                34       34           
  Partials                               7        7           

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