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/upload-tfe-file.php

@@ -0,0 +1,12 @@
+<?php
+/**
+ * upload-tfe-file.php (admin)
+ *
+ * Admin-gated wrapper for the upload-tfe-file fragment.
+ */
+require_once __DIR__ . '/../../bootstrap.php';
+require_once __DIR__ . '/../../src/AdminAuth.php';
+App::boot();
+AdminAuth::requireLogin();
+
+require_once __DIR__ . '/../partage/upload-tfe-file.php';