fix: req annexes, add HTMX inline file validation (MIME/size)

- Annexes file input now required when 'has_annexes' checkbox is checked
- PHP-side validation: if has_annexes but no files, throw error
- HTMX inline file validation: POSTs to validate-file-fragment on file change
  - Validates MIME type against per-field whitelists (couverture, note_intention,
    tfe, annexes)
  - Validates file size with PDF-specific 100MB limit
  - Supports both single-file and multi-file inputs
  - Returns green ✓ or red ✕ inline validation messages
- Shared validation logic in src/Controllers/validate-file-fragment-shared.php
- Admin wrapper: admin/validate-file-fragment.php (with AdminAuth guard)
- Partage route: /partage/validate-file-fragment (dispatched via index.php)
- CSS: .file-validation-msg, .fv-ok (green), .fv-error (red)
- file-field.php: accepts $fieldName for per-input validation type,
  auto-detects admin/partage validate URL

TODO.md

@@ -34,3 +34,8 @@
 - [x] Relax 3-keyword minimum: admin mode (create) requires 1+, edit requires 1+, student (partage) requires 3
 - [x] Add CSS for file preview items (.fp-item, .fp-thumb, .fp-icon, .fp-meta, .fp-name, .fp-size) so annexes/cover/note-intention previews wrap and display correctly
 - [x] Fix TFE file input accept attribute to include video/audio/archive extensions
+- [x] Make annexes file input required when "Ce TFE comporte des annexes" is checked
+- [x] Add PHP-side validation: if has_annexes checked but no annexe files provided, throw error
+- [x] Add HTMX inline file validation: MIME type + file size checked on change via validate-file-fragment endpoint
+- [x] Create shared validation logic (validate-file-fragment-shared.php) used by both admin and partage
+- [x] Add CSS for .file-validation-msg, .fv-ok, .fv-error inline validation messages