* API - Reject broken Jinja2 in url, link_to_open, body and headers, shared validation with the edit form
The edit form validated Jinja2 templates on save but the API did not, so a broken body or
header template saved through the API only failed at fetch time, on every check.
- New jinja2_custom/validate.py: validate_jinja2_fields() renders each value that matches
JINJA2_MARKER_PATTERN and returns {field: translated error}
- The edit form uses it instead of four copies of the same try/except
- API create and update return 400 naming the field
Translations: messages moved to the new file (msgids unchanged); also translated the header
error for fr/it and fixed the cs/es wording.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
* rebuild
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Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
