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>