fix(editor): make Ctrl/Cmd+S save from inside the embedded editor

[erseco]

Stack 3/5, based on #125.

What

The bridge injected by EditorHtmlService swallows Ctrl/Cmd+S and posts REQUEST_SAVE to the parent, but nothing in the view page listened, so the shortcut did nothing. EditorEmbed now re-emits it as save-requested, and ElpxViewPage runs the same save() as the button, guarded by canShowSaveButton and an in-flight save.

No component test: Vitest has no Vue plugin configured. The wiring is two lines, and PR 5 covers the EditorFrame.onMessage path it relies on.

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 (cb54da0) to head (afc90bf).

Additional details and impacted files

@@                   Coverage Diff                   @@
##             fix/viewer-lifecycle     #126   +/-   ##
=======================================================
  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           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.