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/src/Controllers/ThesisEditController.php

@@ -411,8 +411,9 @@ class ThesisEditController
             }
         }
 
-        // ── New TFE files upload ─────────────────────────────────────────────
-        if (!empty($files['files']['name'][0])) {
+        // ── New TFE files upload (from session via HTMX incremental upload) ──
+        $sessionUploads = $_SESSION['tfe_uploads'] ?? [];
+        if (!empty($sessionUploads)) {
             // Count existing TFE files to determine starting number
             $tfeCount = 0;
             foreach ($existingFiles as $f) {
@@ -420,9 +421,9 @@ class ThesisEditController
                     && !str_starts_with($f['file_path'] ?? '', 'http')) {
                     $tfeCount++;
                 }
-                // Don't count captions as separate TFE entries — they'll be renumbered
             }
-            $this->handleTfeFiles($thesisId, $files['files'], $folderPath, $filePrefix, $post, $tfeCount + 1);
+            $this->handleTfeFilesFromSession($thesisId, $sessionUploads, $folderPath, $filePrefix, $tfeCount + 1);
+            $this->cleanupSessionUploads();
         }
 
         // ── New annexe files upload ────────────────────────────────────────────