feat: make speaker notes translatable (fixes #1390)

[hazelzimuto]

Description
This PR resolves the issue where Speaker Notes strings were hardcoded in theme/speaker-notes.js, making them inaccessible to the mdbook-i18n-helpers gettext translation workflow.

Approach
Because xgettext extraction natively targets the markdown content, I integrated the strings into the existing translation pipeline:

Preprocessor HTML Injection: Modified mdbook-course to append a hidden div with the id speaker-notes-translations containing the required UI strings to the end of each chapter's content stream.

Translation Extraction: Because the strings are now embedded in the markdown stream, mdbook-i18n-helpers will automatically extract them into messages.pot for translators.

Dynamic UI Updates: Updated theme/speaker-notes.js to look for this injected div. If found, it dynamically extracts the translated textContent and updates the UI buttons and headings accordingly, falling back to English gracefully if not found.

This ensures translators can continue using their standard .po files without needing to touch JavaScript files.

Strings localized:

"Close speaker notes"

"Speaker Notes"

"Speaker Notes for"

"Could not open popup, please check your popup blocker settings."

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.