refactor: session-based incremental TFE upload via HTMX, drop SortableJS

Replace the client-side FileArray + Sortable drag-to-reorder with a
server-side session-based upload flow:

- New endpoints: /partage/upload-tfe-file, /partage/remove-tfe-file
  (and /admin/ variants) — single-file incremental upload via HTMX
  multipart/form-data with progress bar support
- Session storage: uploaded files go to STORAGE_ROOT/uploads/{session_id}/
  with metadata in $_SESSION['tfe_uploads']
- file-upload-queue.js reduced to single-file previews only (couverture,
  note_intention, annexes thumbnails)
- ThesisFileHandler gains handleTfeFilesFromSession + writeTfeFileFromSrc
  + cleanupSessionUploads for final commit from session temp
- Sortable.min.js removed from all script tags; drag handles and ghost
  CSS removed
- No file_orders[]/file_labels[] hidden field injection needed
- Upload queue survives page refresh (server-owned list)

This eliminates the SortableJS dependency entirely while keeping the
same UX: pick files, see them in a queue, remove individual files.

app/templates/partials/form/fieldset-files.php

@@ -33,7 +33,7 @@ $adminMode = $adminMode ?? false;
     include APP_ROOT . '/templates/partials/form/file-field.php';
     ?>
 
-    <!-- TFE files — multi-file, sortable, with per-file labels -->
+    <!-- TFE files — multi-file, with per-file labels -->
     <div class="admin-form-group admin-files-fieldgroup">
         <label>TFE (obligatoire) :</label>
         <div class="admin-file-input">
@@ -46,9 +46,9 @@ $adminMode = $adminMode ?? false;
                 Les fichiers <code>.vtt</code> sont des sous-titres et seront associés automatiquement à la vidéo précédente.
             </small>
 
-            <!-- Sortable file queue — populated by JS -->
-            <ul id="tfe-file-queue" class="tfe-file-queue sortable-list" aria-label="Fichiers sélectionnés (réordonnable)">
-                <!-- Items injected by file-upload-queue.js -->
+            <!-- File queue — populated by JS -->
+            <ul id="tfe-file-queue" class="tfe-file-queue" aria-label="Fichiers sélectionnés">
+                <!-- Queue rendered server-side via HTMX (upload-tfe-file) -->
             </ul>
             <p id="tfe-file-queue-empty" class="tfe-queue-empty">Aucun fichier sélectionné.</p>
         </div>

app/templates/partials/form/file-field.php

@@ -31,14 +31,16 @@ if (defined('ADMIN_MODE') && ADMIN_MODE) {
 ?>
 <div>
     <label for="<?= htmlspecialchars($id) ?>"><?= htmlspecialchars($label) ?><?= $required ? ' <span class="asterisk">*</span>' : '' ?></label>
-    <div class="admin-file-input"
+    <!-- HTMX validation: scoped to this field -->
+    <form class="admin-file-input file-validation-form"
          hx-post="<?= htmlspecialchars($validateUrl) ?>"
          hx-encoding="multipart/form-data"
          hx-trigger="change from:input[type='file']"
          hx-target="find .file-validation-msg"
          hx-swap="innerHTML"
-         hx-include="[name='admin_mode'], [name='edit_mode'], [name='field_name']">
+         hx-sync="replace">
         <input type="hidden" name="field_name" value="<?= htmlspecialchars($fieldName) ?>">
+        <input type="hidden" name="admin_mode" value="<?= ($adminMode ?? false) ? '1' : '0' ?>">
         <input type="file"
                id="<?= htmlspecialchars($id) ?>"
                name="<?= htmlspecialchars($name) ?><?= $multiple ? '[]' : '' ?>"
@@ -51,7 +53,7 @@ if (defined('ADMIN_MODE') && ADMIN_MODE) {
         <?php if ($hint): ?>
         <small><?= $hintRaw ? $hint : htmlspecialchars($hint) ?></small>
         <?php endif; ?>
-    </div>
+    </form>
 </div>
 <?php
 unset($accept, $hint, $hintRaw, $required, $multiple, $id, $fieldName, $previewId, $validateUrl);

app/templates/partials/form/form.php

@@ -31,7 +31,7 @@
  *   bool   $showFlash           — render flash banners (error/warning/success)
  *   bool   $showContact         — Contact checkbox fieldset
  *   bool   $showCoverPreview    — cover image preview + remove checkbox
- *   bool   $showExistingFiles   — existing thesis files list (sortable, deletable)
+ *   bool   $showExistingFiles   — existing thesis files list (deletable)
  *   bool   $showBackoffice      — Backoffice fieldset (context_note, jury_points, remarks, baiu_link, exemplaires, contact_interne, is_published)
  *   bool   $showEmailConfirmation — E-mail de confirmation fieldset
 
@@ -340,7 +340,7 @@ $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
 
         <!-- Edit-only: existing files management -->
         <div id="edit-existing-files-block">
-            <!-- Existing thesis files — sortable, with labels -->
+            <!-- Existing thesis files — with labels -->
             <?php $thesisFilesList = array_values(
                 array_filter(
                     $currentFiles,
@@ -350,10 +350,7 @@ $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
             <?php if (!empty($thesisFilesList)): ?>
             <div class="admin-form-group">
                 <label>Fichiers du TFE existants :</label>
-                <small style="display:block;margin-bottom:var(--space-2xs);color:var(--text-tertiary)">
-                    Glissez-déposez les lignes pour réordonner les fichiers sur la page publique.
-                </small>
-                <ul id="existing-files-sortable" class="admin-file-list sortable-list">
+                <ul id="existing-files-sortable" class="admin-file-list">
                     <?php foreach ($thesisFilesList as $f):
 
                         $fExt = strtolower(
@@ -400,7 +397,7 @@ $checkedFormatsForSiteWeb = $checkedFormatsForSiteWeb ?? [];
                         <input type="hidden" name="file_sort_order[]" value="<?= (int) $f[
                             "id"
                         ] ?>">
-                        <span class="admin-file-drag-handle" title="Réordonner">⠿</span>
+
                         <span class="admin-file-icon-col"><?= $fIcon ?></span>
                         <span class="admin-file-info">
                             <a href="<?= $fLinkHref ?>" target="_blank" rel="noopener" class="admin-file-name">