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/assets/css/form.css

@@ -555,7 +555,8 @@
   border-color: var(--accent-primary);
 }
 
-.sortable-list {
+/* New-file queue items */
+.tfe-file-queue {
   list-style: none;
   margin: var(--space-2xs) 0 0;
   padding: 0;
@@ -565,11 +566,6 @@
   min-width: 0;
 }
 
-/* New-file queue items */
-.tfe-file-queue {
-  min-height: 0;
-}
-
 .tfe-queue-empty {
   font-size: var(--step--2);
   color: var(--text-tertiary);
@@ -591,28 +587,6 @@
   min-width: 0;
 }
 
-.fq-drag-handle,
-.admin-file-drag-handle {
-  cursor: grab;
-  color: var(--text-tertiary);
-  font-size: 1.1rem;
-  line-height: 1;
-  padding: 0 var(--space-3xs);
-  flex-shrink: 0;
-  user-select: none;
-}
-
-.fq-drag-handle:active,
-.admin-file-drag-handle:active {
-  cursor: grabbing;
-}
-
-.fq-ghost,
-.sortable-ghost {
-  opacity: 0.4;
-  background: var(--accent-muted, #f0f0f0);
-}
-
 .fq-icon {
   font-size: 1.3rem;
   line-height: 1;