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/public/admin/add.php

@@ -55,7 +55,7 @@ function wasSelected($key, $value) {
 $isAdmin = true;
 $bodyClass = 'admin-body';
 $extraCss  = ['/assets/css/form.css'];
-$extraJs   = ['/assets/js/sortable.min.js', '/assets/js/file-upload-queue.js', '/assets/js/beforeunload-guard.js'];
+$extraJs   = ['/assets/js/file-upload-queue.js', '/assets/js/beforeunload-guard.js'];
 require_once APP_ROOT . '/templates/head.php';
 include APP_ROOT . '/templates/header.php';
 include APP_ROOT . '/templates/admin/add.php';