fix: add help email, preserve file names on validation error, license fix

The share link (partage) form does not expose a license field and does
not send access_type_id (defaults to 2/Interne). Server-side validation
was unconditionally requiring a license for non-admin submissions,
causing all share link submissions to fail.

Now the license check is gated on adminMode=false AND accessTypeId=1
(Libre), matching the client-side HTMX fragment behaviour in
licence-fragment.php. Also fixed a use-before-definition where
accessTypeId was referenced before being assigned.

Student form improvements:
- Add xamxam@erg.be mailto link at top of form
- On validation error, append "Si le problème persiste, envoyez un
  e-mail à xamxam@erg.be" to the flash message
- Preserve uploaded file names across validation redirects: store in
  session (share_primed_files_<slug>), display as warning on form
  re-render so the student knows which files to re-select

- License: only required for non-admin when access_type_id=1 (Libre),
  not for Interne (2) or Interdit (3). Fixes share link submissions
  failing with "Veuillez sélectionner une licence". Also fixed
  use-before-definition of accessTypeId.

app/src/Controllers/SystemController.php

@@ -375,14 +375,12 @@ class SystemController
         // Storage directory
         $storageDir      = APP_ROOT . '/storage';
         $storageWritable = is_dir($storageDir) && is_writable($storageDir);
-        $bannersDir = $storageDir . '/banners';
         $coversDir  = $storageDir . '/covers';
         $checks['storage'] = [
             'label'  => 'Répertoire storage',
             'status' => $storageWritable ? 'active' : (is_dir($storageDir) ? 'inactive' : 'failed'),
             'detail' => $storageWritable
                 ? implode('  ·  ', array_filter([
-                    is_dir($bannersDir) ? ('banners/ ' . count(array_diff(scandir($bannersDir), ['.','..'])) . ' fichiers') : null,
                     is_dir($coversDir) ? ('covers/ '  . count(array_diff(scandir($coversDir), ['.','..'])) . ' fichiers') : null,
                   ]))
                 : 'Non accessible en écriture',