refactor: drop the unreachable standalone editor page and dead fields

[erseco]

Stack 4/5, based on #126.

What

Nothing links to /apps/exelearning/editor any more: file actions and the New menu open /view?mode=editor.

• /editor route: EditorController::index becomes a redirect to /view?mode=editor that keeps fileId/path, so old links still work.
• Deleted: templates/editor.php, src/editor/editor-page.ts and its Vite entry. That page also had a SAVE_FILE handler that ignored requestId and re-requested a save on every reply.
• Deleted: src/viewer/ViewerToolbar.vue (never imported).
• Dropped unused data:
  • ViewController's editorIframeUrl initial state (the client builds it from the live webroot) and its now-unused IURLGenerator
  • PackageShape.{hasContentXml,hasScreenshot,hintCount}
  • LoadedElpx.contentLength
  • LoadElpxOptions.signal

Behavior change: /apps/exelearning/editor now returns a 303 instead of rendering its own page.

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 (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.
✅ Project coverage is 93.37%. Comparing base (afc90bf) to head (90a25be).

Additional details and impacted files

@@                   Coverage Diff                   @@
##             fix/editor-ctrl-s     #127      +/-   ##
=======================================================
- Coverage                93.79%   93.37%   -0.43%     
+ Complexity                 152      149       -3     
=======================================================
  Files                       22       22              
  Lines                      661      619      -42     
  Branches                    54       53       -1     
=======================================================
- Hits                       620      578      -42     
  Misses                      34       34              
  Partials                     7        7              

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