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

app/src/Controllers/ThesisCreateController.php

@@ -512,6 +512,12 @@ class ThesisCreateController
         $exemplaireErg  = !empty($post['exemplaire_erg']);
         $cc2r = !empty($post['cc2r']);
 
+        // Annexes validation: if has_annexes is checked, at least one annexe file must be provided
+        $hasAnnexes = !empty($post['has_annexes']);
+        if (!$adminMode && $hasAnnexes && empty($_FILES['annexes']['name'][0])) {
+            throw new Exception('Veuillez fournir au moins un fichier d\'annexe.');
+        }
+
         return compact(
             'authorNames',
             'mail',