chore: cover the editor bridge and SW client, tighten tooling, sync docs

[erseco]

Stack 5/5, based on #127.

What

• Tests: new Vitest suites for:

  • EditorFrame: request-id routing, error replies, timeouts, ignoring foreign message sources
  • service-worker-client: activation, shared registration, session messages, posting failures
  • elpx-loader: filename and ETag parsing, fallbacks

  These modules now count toward the 90% coverage threshold, and the codecov.yml ignores for them are removed. Coverage is 98.6% lines and 93.5% branches.

• Routes: the asset route requires a numeric sessionId, so 12abc no longer casts to 12.

• Biome: biome.json now points at the installed 2.5.14 schema.

• Docs:

  • AGENTS.md and DEVELOPMENT.md now have the right SW path, skill name, save flow and sandbox flags, and no longer mention the unused @nextcloud/viewer.
  • Stale comments about webpack, the Viewer handler and /editor are fixed.

Not in scope

Choosing between Biome and ESLint, since both lint src/ today.

Verification

composer install                 exit 0
npm install                      exit 0
npm run typecheck                exit 0
npm test                         Tests 143 passed (143)
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 (115 tests, 311 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.
⚠️ Please upload report for BASE (refactor/remove-dead-code@90a25be). Learn more about missing BASE report.

Additional details and impacted files

@@                     Coverage Diff                      @@
##             refactor/remove-dead-code     #129   +/-   ##
============================================================
  Coverage                             ?   93.81%           
  Complexity                           ?      149           
============================================================
  Files                                ?       25           
  Lines                                ?      776           
  Branches                             ?       89           
============================================================
  Hits                                 ?      728           
  Misses                               ?       35           
  Partials                             ?       13           

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