…oss codebase (#4661)

# Description of Changes

**What was changed**
- Fixed a CSS typo in `imageHighlighter.css` (`visbility` →
`visibility`) to restore the intended transition behavior.
- Corrected spelling/grammar in logs, comments, and console messages:
  - `PdfUtils.java`: log message “overlayed” → “overlaid”.
  - `CustomPDFDocumentFactoryTest.java`: comment typo “neeed” → “need”.
  - `FlattenController.java`: comment “readd” → “re-add”.
  - `PdfContainer.js`: console log “Hidding” → “Hiding”.
  - `navbar.js`: clarified tooltip comment wording.
  - `languages.html`: corrected Vietnamese label to “Tiếng Việt”.
- `ConvertWebsiteToPdfTest.java`: converted/de-duplicated comments to
clear English and improved assertion messages.
- No functional code paths altered except the CSS fix; the rest are
non-functional text improvements (logs/comments/tests/labels).

**Why the change was made**
- The CSS typo prevented the `visibility` transition from working as
intended.
- Consistent, correct wording improves developer experience, test
readability, and UI polish (language list).

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## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/DeveloperGuide.md#6-testing)
for more details.

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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Reece Browne <74901996+reecebrowne@users.noreply.github.com>