Add embedded static eXeLearning editor support

[erseco]

Summary

This PR adds a fully integrated embedded eXeLearning editor that runs inside Moodle, allowing teachers to create and edit web content without leaving the platform.

Note: This PR replaces #25 with a cleaner single-commit history.

Features

• Editor bootstrap (editor/index.php, editor/static.php, editor/save.php) — loads the static editor build inside an iframe with a postMessage bridge for save/load communication
• Admin settings widget — install/update/uninstall the editor from GitHub releases with spinner UX and multilingual status messages
• External API (manage_embedded_editor) — AJAX endpoints for install and status actions
• Source resolver — moodledata > bundled precedence policy for editor assets
• PostMessage bridge (moodle_exe_bridge.js) — handles OPEN_FILE, REQUEST_EXPORT, and document change tracking via yjs
• Admin-aware error messages — admins see "install from plugin settings", non-admins see "contact your administrator"
• Blueprint — Playground configuration with all default settings and sample activities
• Translations — en, es, ca, eu, gl
• Capability — mod/exeweb:manageembeddededitor
• CI — Playground PR preview support, editor release checker

Technical notes

• Trusted origins for postMessage use scheme+host only (not full wwwroot path) to work on subpath deployments
• Editor errors render as inline HTML pages (not moodle_exception) to display cleanly inside iframes
• Includes manage_embedded_editor_upload.php endpoint for Playground environments where PHP WASM cannot directly fetch GitHub

Test plan

• Go to Site administration > Plugins > eXeLearning (website) settings
• Set editor mode to "Integrated editor (embedded)"
• Click "Install latest version" — verify spinner + "Downloading and installing..." message
• Create a new eXeLearning web activity
• Click "Edit with eXeLearning" — verify editor opens
• Make changes, save — verify content updates
• Test as non-admin: verify "contact your administrator" message when editor not installed
• Verify translations display correctly (switch language)

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Moodle Playground Preview

The changes in this pull request can be previewed and tested using a Moodle Playground instance.

⚠️ The embedded eXeLearning editor is not included in this preview. You can install it from Modules > eXeLearning Web > Configure using the "Download & Install Editor" button. All other module features (ELPX upload, viewer, preview) work normally.