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/file-field.php

@@ -31,14 +31,16 @@ if (defined('ADMIN_MODE') && ADMIN_MODE) {
 ?>
 <div>
     <label for="<?= htmlspecialchars($id) ?>"><?= htmlspecialchars($label) ?><?= $required ? ' <span class="asterisk">*</span>' : '' ?></label>
-    <div class="admin-file-input"
+    <!-- HTMX validation: scoped to this field -->
+    <form class="admin-file-input file-validation-form"
          hx-post="<?= htmlspecialchars($validateUrl) ?>"
          hx-encoding="multipart/form-data"
          hx-trigger="change from:input[type='file']"
          hx-target="find .file-validation-msg"
          hx-swap="innerHTML"
-         hx-include="[name='admin_mode'], [name='edit_mode'], [name='field_name']">
+         hx-sync="replace">
         <input type="hidden" name="field_name" value="<?= htmlspecialchars($fieldName) ?>">
+        <input type="hidden" name="admin_mode" value="<?= ($adminMode ?? false) ? '1' : '0' ?>">
         <input type="file"
                id="<?= htmlspecialchars($id) ?>"
                name="<?= htmlspecialchars($name) ?><?= $multiple ? '[]' : '' ?>"
@@ -51,7 +53,7 @@ if (defined('ADMIN_MODE') && ADMIN_MODE) {
         <?php if ($hint): ?>
         <small><?= $hintRaw ? $hint : htmlspecialchars($hint) ?></small>
         <?php endif; ?>
-    </div>
+    </form>
 </div>
 <?php
 unset($accept, $hint, $hintRaw, $required, $multiple, $id, $fieldName, $previewId, $validateUrl);