feat(admin): add tessdata language management for OCR and download support (#5519)
# Description of Changes
### What was changed
- Added new admin-only API endpoints to:
- List installed tessdata OCR languages
- Fetch available tessdata languages from the official Tesseract
repository
- Download selected tessdata language files directly into the configured
tessdata directory
- Implemented server-side validation, safe language name handling, and
directory writability checks.
- Extended the Admin Advanced Settings UI to:
- Display installed tessdata languages
- Show available remote languages not yet installed
- Allow selecting and downloading additional languages via a
multi-select UI
- Gracefully fall back to manual download links when the tessdata
directory is not writable
- Added new i18n strings for all related UI states (loading, success,
error, permission warnings).
### Why the change was made
- Managing OCR languages previously required manual filesystem
interaction.
- This change improves usability for administrators by enabling in-app
management of tessdata languages while maintaining security constraints.
- The writable directory check and manual fallback ensure compatibility
with restricted or containerized environments.
<img width="1282" height="832" alt="image"
src="https://github.com/user-attachments/assets/aa958730-0ffb-4fd6-9af8-87c527a476e4"
/>
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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)
### Translations (if applicable)
- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)
### 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>
